Resident peritoneal macrophages (PEM) harvested from female B6C3F1 mice given an intraperitoneal injection of ammonium metavanadate (2.5 or 10 mg V/kg), an equivalent amount of ammonium in the form of ammonium chloride, or sodium phosphate buffer (0.1 M, pH 7.2) every third day for 6 weeks, were subjected to flow cytometric analysis of Fc tau 2a and Fc tau 2b receptor expression, and photometric microassay to measure receptor mediated binding and phagocytosis of sheep red blood cells (SRBC). The NH4Cl and 10V groups showed 21.7 and 17.2% lower mean fluorescence channel (MFC) values and 7.1 and 5.9% lower values in percentage fluorescence-positive cells than the phosphate buffer control with respect to Fc tau 2a expression. For Fc tau 2b expression, the 10V group showed significantly (P less than 0.05) lower MFC (31.2%) and percentage fluorescence-positive cells (15.7%) than the phosphate buffer control. Though the four groups did not show a significant difference in Fc tau 2a mediated binding and phagocytosis of SRBC, the 10V group showed a significantly lower Fc tau 2b mediated binding and phagocytosis. The results indicate that the reduction in Fc tau 2b expression and function could contribute toward the previously observed depression in phagocytosis, NADPH-oxidase and superoxide generation in peritoneal macrophages obtained from vanadate-treated animals.