Incubation time with N-acetylsphingosine required for significant inhibition of PMN phagocytosis. PMNs were aliquoted into tubes and N-acetylsphingosine (10 μmol/L) added at the indicated time points. In all cases, PMNs were activated with FMLP 10 minutes before the addition of EIgG as indicated by the arrow. Time 0 is the time of EIgG addition; times < 0 are after EIgG addition, and times < 0 are during preincubation with N-acetylsphingosine. Significant inhibition occurred even when N-acetylsphingosine was added after phagocytosis had been initiated by the addition of EIgG. Values represent the mean ± SD for three experiments. The phagocytic index in the absence of N-acetylsphingosine (100% control) was 145 ± 46.