Zinc chloride (0.9 mM)--an inhibitor of the processing of the initial polypeptides of Picornaviridae--was used to accumulate large precursors of echovirus 33 (EV33) and notably two proteins of 147- and 97-kDa. Polyclonal hyperimmune sera were raised in mice against these polypeptides and assayed by immunoblotting against EV33-infected cells blocked or not with ZnCl2, showing that protein 147--2ABC3ABCD according to the L434 convention--can be considered to be the precursor of the two non-structural proteins P2 and P3. Sixty-three serum specimens from subjects exhibiting varying antibody status against EV33 by seroneutralization were investigated by immunoblotting against an EV33 ZnCl2-blocked antigen. Some subjects infected with EV33 were shown to elicit antibodies which recognize the non-structural precursor polypeptides, a fact whose clinical significance needs further evaluation.