Polymorphonuclear leukocytes (PMN) from healthy, normal control cats and FeLV-infected cats were analyzed for differences in phagocytic capabilities. One week after experimental infection, PMNs from FeLV-infected cats exhibited a marked decrease in phagocytic function as determined by the luminol-dependent chemiluminescent response. Depressed PMN function was observed in these cats during the viremic stage of infection and subsequently remained depressed after the cessation of viremia. The data presented here suggest that while nonviremic cats are reported as clinically normal, they may in fact be exhibiting a suppressed PMN function.