PAbl614, an SV40-speciíic monoclonal antibody of the panel PAbl601-1636 reacts with large and small tumor antigens of SV40, BK and JC virus, and with polyoma virus large and middle tumor antigens, but not with the large tumor antigen of the lymphotropic papova virus. Using immunofluorescence and immunoblot competition assays and ELISA with synthetic peptides, it is shown that the epitope is represented by the SV40 tumor antigen undecapeptide, K39-E49. This peptide comprises the tumor antigen consensus sequence, H42-G47, of the polyoma viruses. However, the epitope of PAbl614 probably does not exactly coincide with this hexapeptide. This explains why some cross-reactions are less strong, or absent, as in the case of the lymphotropic papova virus. Further antibodies of the PAbl601-1636 panel that cross-react with the JC virus large tumor antigen are PAM602,1604,1606,1618,1621,1622,1623, 1624,1626,1629, and 1633.