Two monocyte subsets from the peripheral blood of healthy humans were isolated by countercurrent centrifugal elutriation (CCE), and their functional heterogeneity was assessed. Cell volume of the smaller monocyte population was (332 +/- 32 mu 3), compared with the larger monocytes (380 +/- 30 mu 3). Superoxide release by large monocytes exposed to opsonized zymosan was 1.5-fold more active than those of small monocytes. Furthermore, the production of colony-stimulating factor (CSF) and myeloperoxidase activity by the larger monocytes was 2-fold and 1.4-fold greater than that of small monocytes, respectively. Thus, CCE provides a useful means to isolate 2 subsets of monocytes with similar functional properties but quantitatively different activities. This suggests that the 2 subsets may be closely related in their developmental spectrum.