The effect of viral infections on insulin binding in vivo was evaluated by measuring the binding of 125I-insulin to several different tissues. We found that splenic leucocytes from mice infected with either the diabetogenic (D) or non-diabetogenic (B) variants of encephalomyocarditis virus, herpes simplex virus, or lactic dehydrogenase virus showed up to a 130% increase in insulin binding. As much as a 300% increase in the binding of 125I-insulin to splenic leucocytes was observed in mice given bacterial lipopolysaccharide. In neither virus-infected nor lipopolysaccharide-treated mice was there any substantial change in insulin receptors on thymocytes, liver membranes, or peripheral erythrocytes. Thus, the increased binding of insulin appears to be limited to leucocytes and does not appear to represent a generalized metabolic alteration. These experiments suggest that during infection, the binding of insulin to leucocytes, which is widely used to measure insulin receptors, may not always accurately reflect the insulin receptor status of other tissues.