Solcoseryl is a protein-free, standardized dialysate/ultrafiltrate derived from calf blood, which has been shown to improve situations of impaired healing in both experimental animals and man. Its activity seems to be multifactorial although the precise cellular and molecular mechanisms contributing to its effect have not been fully elucidated. Since monocyte-derived macrophages play a central role in inflammation and particularly in wound healing and tissue remodelling, the effect of the dialysate on human monocytes cultured in vitro for 10 days in the presence of human serum was studied. The results show that the drug, at concentrations ranging from 0.2 to 2%, increases the cell density of the cultures and the cell protein content, and favours the differentiation of monocytes into macrophages when 1% serum concentration is used in the culture medium. These effects are no more apparent when the serum concentration was raised to 10%. These data suggest that the drug may substitute, at least in part, for serum in monocyte-macrophage cultures. The observed effects give a sound basis for at least a partial explanation of the therapeutic effects of the drug, particularly at sites where the supply of serum-derived factors is limited.