Abstract We investigated modulation of p60 src expression in human mononuclear phagocytes. By analysis of [ 35S]methionine-labelled cells we found that synthesis of p60 src is higher in human monocytes compared to macrophages derived from in vitro cultivation of monocytes. Western blot analysis showed that expression of p60 src in monocyte-derived macrophages can be enhanced if monocytes are differentiated into macrophages in the presence of interferon-γ (IFN-γ), or tumor necrosis factor-α (TNF-α). Enhanced p60 src expression caused by IFN-γ or TNF- a correlated with an enhanced autophosphorylating kinase activity assayed in anti-p60 src immune precipitates. In vivo phosphorylation of p60 src and analysis of phosphopeptides by tryptic digestion showed that treatment with cytokines did not affect the pattern of phosphorylation of distinct phosphopeptides. The human monocytic cell lines, U937 and HL-60, induced to differentiate along the monocytic pathway by IFN-γ, or a combination of IFN-γ and TNF-α, expressed higher amounts of the p60 src, but not of the p59 fyn or p62 yes, kinase activity. These findings show that p60 src is modulated in the course of differentiation of human monocytes to macrophages, and that macrophage-activating cytokines increase p60 src expression in human monocyte-derived macrophages.