The effect of concanavalin A (Con A) on the course of early infection of HeLa cells with purified radioactive human rhinovirus type 2 (HRV-2) or poliovirus type 2 (P-2) has been examined. Several early steps in infection were inhibited before the uncoating of parental virus. Con A, at 100 mug/ml, reduces attachment of virus when added to cells before infection. Con A also detectably slows the normal progression of adsorbed virus to tightly bound forms characterized, in the case of HRV-2, by resistance to elution by EDTA, or in a case of P-2, by isolation of a characteristic virus-membrane complex. When Con A is added together with either virus, it also inhibits cell-mediated eclipse of infectivity and the formation of non-infective subviral particles.