African green monkey (AGM) B-lymphotropic papovavirus (LPV) particles were separated from infected B-lymphoma BJA-B cells by neuraminidase and deoxycholate treatment and were further purified twice by CsCl density centrifugation. The SDS-PAGE analysis of virus particles banding at a density of 1.3510 g/ml and radioactively labeled by 125I revealed a major polypeptide of 40,000 and two minor polypeptides of 42,000 and 29,000. In addition, in infected BJA-B cells, all three viral structural polypeptides could be identified by immunoprecipitation. A nonstructural phosphopolypeptide of approximately 90,000 MW could be detected when sera against SDS-denatured SV40 T antigen or an AGM serum pool were used for immunoprecipitation. It is suggested that the 90K polypeptide of LPV represents an equivalent to T antigens of other papovaviruses.