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Eight independent cell lines, derived from human testicular germ-cell tumors, were examined for the expression of various markers. These included major histocompatibility and embryonic antigens, chorionic gonadotropin, alpha fetoprotein, alkaline phosphatase, plasminogen activator, and infectivity by SV40. No line consisted primarily of choriocarcinoma or yolk sac cells, but several contained cells resembling murine embryonal carcinoma; some of these lines formed tumors with the distinctive features of embryonal carcinoma when injected into immunosuppressed animals. It is proposed that human embryonal carcinoma cells, unlike those of the mouse, correspond to a preblastocyst stage of development.

A cloned human embryonal carcinoma (EC) cell line has been derived from a testicular teratocarcinoma, and r e producibly forms EC tumors when injected into athymic (nuhu) mice. These human EC cells are characterized by a newly described stage-specific embryonic antigen, SSEA-3. Unlike murine EC cells, they express major. histocompatibility antigens (HLA-A, 8, C and µ-globulin) but do not express the embryonic antigen SSEA-I. We also report that these cells appear to be capable of differentiation and that this can be induced by initiating cultures at low cell density. Differentiation is marked by the appearance of morphologically distinct cells and by the induction of SSEA-I, whereas the expression of other antigens, including SSEA-3, is initially diminished. This well-characterized system of human EC cells provides a model for the future investigation of other human teratocarcinoma cell lines and for the analysis of cellular differentiation during early human development.

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A cloned human embryonal carcinoma (EC) cell line has been derived from a testicular teratocarcinoma, and reproducibly forms EC tumors when injected into athymc (nu/nu) mice. These human EC cells are characterized by a newly described stage‐specific embryonic antigen, SSEA‐3. Unlike murine EC cells, they express major histocompatibility antigens (HLA‐A, B, C and β2‐microglobulin) but do not express the embryonic antigen SSEA‐I. We also report that these cells appear to be capable of differentiation and that this can be induced by initiating cultures at low cell density. Differentiation is marked by the appearance of morphologically distinct cells and by the induction of SSEA‐I, whereas the expression of other antigens, including SSEA‐3, is initially diminished. This well‐characterized system of human EC cells provides a model for the future investigation of other human teratocarcinoma cell lines and for the analysis of cellular differentiation during early human development.

Serial undiluted passage of clonally purified herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) in 4 different types of cells resulted in (i) partial loss (84–98%) of infectivity and (ii) the appearance of a new DNA within the infected cells with a higher buoyant density in CsCl (p = 1.732 g/cm³) than that of HSV-1 (1.725 g per cm³) or that of HSV-2DNA (1.727 g/cm³). Serial diluted passages of these stocks restored the high infectivity titers with a concomitant loss of the high density DNA within the infected cells. Serologic studies and electron microscopy indicated that the HSV stocks were not contaminated with an extraneous virus which might yield a DNA with the high buoyant density. Characterization of purified HSV from the serially passed undiluted stocks indicates that the high density DNA is a part of the HSV virion replicating under these conditions. The results obtained suggest the production of defective interfering particles after undiluted passage of HSV.

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