Human monocytes cultured in vitro exert cytostatic activity against the non-adherent human leukemic cell line K-562. K-562 cells in asynchronous growth were added as target cells to adherent mononuclear blood cells, either freshly isolated or cultured in vitro for 4 and 12 days. After 24 hours of co-culture, the non-adherent cells were gently removed and incubated further. Target cell DNA-synthesis and cellular DNA content were examined by methyl-3H-thymidine incorporation and by flow cytofluorometry, respectively. Cell-proliferation curves were obtained. Monocytes cultured in vitro exert a reversible cytostatic activity, which is a partial cell cycle block in the G0/G1 phase. Target cells exposed to freshly isolated monocytes also showed accumulation of cells in the G0/G1 phase. These target cells did not regain their normal proliferation rate before day 3 after removal from the monocyte cultures. This effect may, however, to some extent be due to loosely adherent mononuclear blood cells removed from the monocyte cultures together with the target cells.