By immunofluorescence staining, a specific surface antigen induced by Venezuelan equine encephalomyelitis virus was detected on L-929 cells. Formation of the antigen was independent of viral ribonucleic acid synthesis.Deoxyribonucleic acid (DNA) (2,4,8,9,11,12,17) (Fig. 1); uninfected cells did not fluoresce. The specificity of the staining was confirmed by the absence of surface fluorescence on cells infected with VEE virus and stained with variola, chikungunya, or yellow fever fluorescein-conjugated antiviral serum. Similar results were obtained when cell monolayers were infected with any of these three viruses and stained with conjugated VEE antiserum. Further incidence of staining specificity was obtained by demonstrating that cell surface antigen fluorescence was blocked when cells were first treated with unlabeled monkey antiviral serum prior to staining with conjugated antiviral serum. Induction of cell-surface antigen was not limited as to the source of virus, the strain of virus, or the L-929 cell line; the phenomenon was elicited by both infective mouse brain and chick embryo preparations, by two different attenuated VEE virus strains (1, 7), and on guinea pig lung, McCoy, and baby hamster kidney (BHK21 /C13) cell lines. Induction of cell surface antigen by VEE virus is dependent on the presence of infectious virus particles. Virus inactivated by either ultraviolet irradiation or heat (56 C, 16 hr) failed to induce the formation of cell surface antigen. Furthermore, the number of cells showing surface antigen was reduced approximately 93% when virus was mixed with antiviral serum before it was added to cell monolayers. Additional evidence that the manifestation of cell surface antigen is associated with the presence of infectious virus particles was noted by the demonstration of a linear relationship between relative virus concentration and the number of cells showing surface antigen. This finding suggests that each cell exhibiting fluorescent surface antigen results from infection by a single infective virion or aggregate not divisible by dilution. The dependency of surface antigen induction on viable virus is consistent with observations on the direct association between infection or production (or both) of Epstein-Barr virus and cell membrane antigen (4).To compare the rate of appearance of fluores-713