Single radial hemolysis (SRH), neutralization (NT), and hemagglutination inhibition (HI) tests were carried out on sera from horses immunized against the Prague and Miami strains of equine influenza virus. The HI and NT tests demonstrated good sensitivity; the sensitivity of the SRH test was somewhat lower. The NT titers of individual sera were correlated very closely with the HI titers, although the NT titers were higher. SRH zone diameters of individual sera also showed significant correlation with the NT and NI titers. The SRH test appears to be suitable for large-scale serological surveys and offers the advantages of rapidity and simplicity.

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The production of the virus-inhibiting factor or interferon (IF) was highest in cells incubated at 37 C after inoculation with Newcastle disease (ND) virus and decreased as the incubation temperature was lowered. Shift-down of incubation temperature to 32 C or 34 C after incubation at 37 C for 4-7 hr enhanced IF production in cell cultures stimulated with ND virus, as compared. with cultures incubated continuously at 37 C. Shift-down to 32 C after incubation at 37 C for 6 hr. was optimal for this enhancement of IF yield. Enhanced IF production was also observed in cell cultures irradiated by ultraviolet light 4-7 hr after stimulation with ND virus.Recent results of clinical studies (7) and considerable experimental evidence (4) have produced potent IF in human diploid cells by the superinduction process, in which IF production with synthetic double-stranded RNA, polyinosinate-polycytidylate, is enhanced by the use of metabolic inhibitors (3, 9, 10).In the present study we have investigated the production of IF in human diploid cells by stimulation with Newcastle disease (ND) virus to define more optimal conditions for IF preparation. The production of IF in these cells stimulated with ND virus was enhanced by shift-down of the incubation temperature to 32 C or 34 C after incubation at 37 C for 4-7 hr, as compared with cultures incubated continuously at 37 C. Enhanced IF production was also observed in cell cultures irradiated by ultraviolet light 4-7 hr after stimulation with ND virus. This paper presents these observations. 907

Interferon (IFN) yield after superinduction declined gradually by in vitro senescence of the cells, while it remained unchanged in IFN-pretreated cells. IFN yield after stimulation with Newcastle disease virus remained unchanged by in vitro senescence.

SummaryUltraviolet irradiation of cells after stimulation with polyinosinate-polycytidilate markedly enhanced and prolonged the production of interferon in human diploid fibroblasts. It further boosted the interferon production, when applied to cells primed with interferon.