Spontaneously proliferating human leucocytes have been characterized and enumerated using multiparameter flow cytometry. The frequency of spontaneously proliferating cells amongst human peripheral blood mononuclear cells, was determined on the basis of BrdUrd incorporation and total DNA content in samples of cells incubated in medium without added mitogen for several days. The frequency of proliferating cells decreased from an initial level of approximately 6 x 10(-4) to 3 x 10(-4) after 30 h of incubation, and then rose to approximately 2 x 10(-2) after 100 h of incubation. In one reference person, the frequency showed only minor variation in this pattern over a 1.5 year interval. Simultaneous measurement of proliferation and determination of immunological subclass, as indicated with Hoechst 33342 staining and surface markers, showed an over-representation of CD19-positive cells, compared with CD2-positive cells and subsets of CD2-positive cells (CD4-positives and CD8-positives). This method can be used as an indicator of exposure to agents when results from animal tests are to be compared with results from human populations. The advantages are that no cell culturing is needed to perform the test, it provides the possibility of further characterizing proliferating cells, and the rapid flow-cytometric enumeration.