Previous experiments have shown that population average surface IgG content is correlated with the specific antibody production rates of batch hybridoma cultures. Therefore, surface associated IgG content of single hybridoma cells might indicate antibody secretion rates of individual cells. Moreover, the surface IgG content should reflect the pattern of secretion rates during the cell cycle. To probe for IgG secretion rates during the cell cycle, a double staining procedure has been developed allowing simultaneous flow cytometric analysis of surface IgG content and DNA content of murine hybridoma cells. Crosslinking of the surface associated immunofluorescence with the cell by paraformaldehyde fixation permits subsequent DNA staining without loss of immunofluorescence. The optimized protocol has been used to determine the pattern of the surface IgG fluorescence as a function of the cell cycle position. It is highest during the G2 + M cell cycle phase and the experimental data are in excellent agreement with the previously predicted secretion pattern during the cell cycle.