The effect of pig monocytes (MNs) on the baseline frequency of sister chromatid exchanges (SCEs) and cell-cycle progression of pig lymphocytes was studied in plasma leukocyte (PLCs) and whole blood leukocyte cultures (WBCs). No variation in SCE frequency was observed between control WBC and PLC, nor did the addition of pig MNs to PLCs modify the baseline frequency of SCEs. Cell proliferation was slower in PLCs than in WBCs. Variations in cell-cycle progression of pig lymphocytes from PLC were observed both in the absence and presence of adherent cells in the culture. In MN-free cultures, lymphocytes proliferated foster than in parallel PLC cultures. However, when MNs were seeded into the cultures, cell-cycle progression gradually slowed as a function of the concentration of adherent cells present in the cultures. This finding shows that pig MNs modulate the in vitro cell-cycle progression of pig lymphocytes in a dose-dependent manner and that the low baseline SCE frequency of pig lymphocytes is independent of the presence or absence of MNs in the culture.