A subline of the P3 (Jijoye) Burkitt lymphoma cell line, designated P3HR-1, initially contained 1 to 5% of cells which were positive by indirect immunofluorescence with selected human sera. After 4 months of propagation, this cell line regularly showed 15 to 40% reactive cells. Antigen(s) in the cell line which was reactive by immunofluorescence was similar or identical to that found in several other Burkitt tumor cell lines in previous studies. When the cells were incubated at 35 or 32 C for 9 to 15 days without refeeding, more than 50% of the cells became immunofluorescence-positive. Thirteen different cultures of P3HR-1 cells, which contained up to 75% immunofluorescence-positive cells, were thin-sectioned and examined by electron microscopy. The percentage of cells containing herpes-type virus particles in the cultures varied from <3 to 78%. There was generally a good correlation between the number of immunofluorescent cells and the number of cells containing virus particles. The number of virus particles per cell section ranged from 1 to more than 100. These results strongly support the hypothesis that the immunofluorescent antigen is related to the presence of the herpes-type virus particle in the cells.