Thymus-derived lymphocytes (T cells) and bone marrow-derived lymphocytes (B cells) from mouse spleens bind specifically to antigen-derivatized nylon fibers. The fiber-bound population consisted of about 60%-70% B cells and 30% T cells as determined by cytotoxicity, fluorescence, and antibody-complement binding assays. Essentially all fiber-bound cells were viable and could be accounted for as T or B cells. Enriched populations of T or B cells could be isolated on the fibers by destruction of one or the other cell type with the appropriate antiserum plus complement. T or B cells could also be fractionated according to their relative affinity (or avidity) for a given antigen.In previous reports, we have described the fractionation of specific antigen-binding cells from lymphoid cell populations of immunized and unimmunized animals by specific attachment to antigen-derivatized fibers (1, 2). Although the isolated lymphoid cells all have specificity for the chosen antigens, it is likely that they are heterogeneous in their function and degree of differentiation. The most conspicuous functional heterogeneity among lymphocytes is reflected by the presence in the population of thymus-derived lymphocytes (T cells) and bone marrow-or bursa-derived lymphocytes (B cells) (3). Additional distinctions must be made between thymocytes and competent T cells. Similarly, B cells include several functional classes, notably antigen-sensitive B cells that can differentiate into secreting cells when triggered by antigen, and the secreting cells or plaque-forming cells themselves.Out studies (1, 2) have indicated that specific antigenbinding cells isolated by fiber fractionation do not include either thymocytes or plaque-forming cells. In the present paper, we provide evidence that antigen-derivatized fibers bind both T and B cells. The cells can be fractionated according to their relative avidities* for antigen and, by the use of appropriate antisera, enriched populations of T or B cells with specificity for a given antigenic determinant can be obtained.