Flow cytometric analyses were performed to study intracellular single-cell calcium transients ([Ca2+]i) in suspended human neutrophils during the initial phase of N-formyl peptide stimulation. Thereby, two neutrophil populations became apparent. Early maximally Ca(2+)-responding (high fluorescence) neutrophils and not-yet Ca(2+)-responding (low fluorescence) neutrophils, but no neutrophils with intermediate levels of [Ca2+]i were detected. Within 7 s the number of low fluorescence neutrophils decreased and the number of high fluorescence neutrophils increased maximally. This suggests that [Ca2+]i transients occurred abruptly in individual neutrophils within a time interval below 1 s. At lower N-formyl peptide concentrations the lag times of individual neutrophils and the interval time of maximal activation of the [Ca2+]i-responding neutrophil population increased, however the percentage of [Ca2+]i-responding cells decreased. Surprisingly, no influence of the N-formyl peptide concentration on the [Ca2+]i-induced fluorescence signal of the individual cell was observed: it was always in an almost maximal range or not responding. In parallel, binding studies performed with fluorescein-labeled N-formyl peptide revealed that the heterogeneity of [Ca2+]i-responding cells cannot be explained by different receptor occupancy. In summary, this study demonstrates that [Ca2+]i transients induced by N-formyl peptides in suspended individual human neutrophils occur very rapidly in an almost "all-or-none manner" and that the mean increasing fluorescence signal of a calcium indicator within a whole neutrophil population results from varying lag times of the individual cells, rather than from the mean simultaneous progress of many cells.