Human parvovirus B19 is a member of the autonomous parvoviridae but in contrast to other members of the genus has not been shown to agglutinate red blood cells. We now report that the virus agglutinates red cells of primate origin, though with plasma-derived virus this activity is masked by the presence of an IgM-like inhibitor. This observation is consistent with the presence on the erythroid precursor target cell of a specific receptor for parvovirus B19.