AbstractHuman peripheral blood monocytes were separated into four different subpopula‐tions by means of a discontinuous bovine serum albumin gradient. Of the least dense population, 7% were present in fraction A, 11% in fraction B, 28% in fraction C and of the most dense, 34% were in fraction D. The rest (17%) of the recovered cells sedimented as a pellet, of which 95% were dead. The monocytes of fraction D (ϱ = ⩾ 1.075 kg/l) were major interleukin 1 (IL1) producers and their presence enhanced immunoglobulin synthesisin vitro. Fraction C (ϱ = ⩾ 1.070 kg/l) were the major prostaglandin E2(PGE2) producers and demonstrated suppressor activity onin vitroIgG and IgM synthesis. Fractions A and B had minimal production of either IL1 or PGE2and lesser effects on the IgG and IgM synthesis. These data demonstrate functional heterogeneity of peripheral blood monocytes with respect to production of both IL1 and PGE2as well as accessory cells for immunoglobulin sy