We investigated with three-color flow cytometry the expression of perforin (Pf) in normal human lymphocyte subpopulations identified by means of activation and differentiation-related antigens. Interestingly, Pf could be detected in a substantial subset (13 +/- 2%) of memory CD4+ CD45RO+ cells, on relevant percentages of memory (CD45RO+) and naive (CD45RA+) CD8+ cells and on virtually all CD3- CD16+, CD3- CD56+ and NKB1+ natural killer cells, as expected. The analysis of fluorescence intensity showed higher levels of Pf expression on CD8dim and NK cells compared to CD8bright and CD4+ lymphocytes, Pf and CD69, HLA-DR, CD95 and CD25 activation/differentiation-related antigens were never co-expressed. On average, 15 +/- 3% of CD3+ CD28+ cells were found to be Pf+, in line with a previously activated or memory cell type. Comparable percentages of CD8+ CD11b- (cytotoxic) and CD8+ CD11b+ (suppressor) T cells were Pf+. Multiparameter flow cytometry is a powerful tool to detect minute fractions of Pf-expressing cells in heterogeneous populations.