Human thrombin has been shown to stimulate monocyte chemotaxis, phagocytosis, and interleukin (IL8) production, but the mechanisms responsible for stimulation are not well defined. In some cells, thrombin stimulation of proliferation appears to require both cleavage of the proteolytically activated receptor for thrombin (PAR1) and activation of a nonproteolytically activated thrombin receptor (N‐PAR), while in others activation of either receptor alone may be sufficient for stimulation. We, therefore, have initiated studies to address thrombin receptor expression and cell responsiveness to thrombin in interferon gamma (IFNγ)‐differentiated and nondifferentiated U937 monocytic cells. Northern blot analysis shows that PAR1 expression is upregulated upon differentiation. Experiments with biotinylated and ¹²⁵I‐thrombin show that specific thrombin binding is dramatically increased by differentiation although it is not clear if this binding is to PAR1 or to a separate binding component such as N‐PAR which is present on fibroblasts and other cells. Addition of thrombin at concentrations of 1–10 μg/ml (30–300 nM, concentrations where specific thrombin binding is observed) stimulates proliferation of IFNγ‐differentiated U937 cells but not of undifferentiated U937 cells. Thrombin also stimulates interleukin‐6 (IL6) production in IFNγ‐differentiated U937 cells. Moreover, thrombin induces high levels of IL6, interleukin‐1β (IL1β), and tumor necrosis factor‐α (TNFα) production by peripheral blood mononuclear cells (PBMC) and monocytes. These results show that differentiated U937 cells and mature PBMC are responsive to thrombin whereas nondifferentiated U937 are not. Further, this responsiveness appears to correlate with expression of PAR1 and to a dramatic increase in specific thrombin binding. That thrombin stimulates cytokine production and proliferation in populations of differentiated monocytes suggests that thrombin may be an important regulator of inflammation and wound healing. J. Cell. Physiol. 177:76–84, 1998. © 1998 Wiley‐Liss, Inc.