To analyse the amounts of various cellular components in cultured mammalian cells by flow cytometry, we tried to reduce the autologous fluorescence in human-human hybridoma HB 4 C 5 cells producing IgM. The autologous fluorescence of the cells decreased after repeated rinsing with phenol red-free MEM medium at 4°C before and after ethanol-fixation. The method could be useful in the analyses of various cellular components by flow cytometry.