Several malignancies over-express the epidermal growth factor receptor, ligation of which results in cellular differentiation and multiplication. Mononuclear phagocytes secrete this cytokine and its receptor has been detected on microglial cells. This communication describes the expression (and its regulation) of epidermal growth factor receptor (EGFR) on U937 cells. We have shown that a few are EGFR-positive, with expression being up regulated by interleukin 6 (IL-6). Also, when cultured in the presence of serum with the monoclonal anti-EGFR, ICR62, U937s showed a reduced growth rate. By contrast, ICR9 caused a significant increase in cellular proliferation. Both antibodies induced cycle arrest in late G₁/S phase. When the cells were cultured in the absence of serum, low antibody concentration (10μg/ml) showed an early inhibitory effect on cell proliferation. By contrast, at high antibody concentrations (50μ/ml), ICR62 significantly increased the proliferation of U937 cells. We suggest that these results provide indirect evidence for an autocrine action of EGF on U937 cells.