Feline immunodeficiency virus (FIV) grown in cat lymphocyte and thymocyte cultures was labelled with L-[35S]methionine or [3H]glucosamine and virus-coded proteins were identified using immunoprecipitation. Polypeptides with apparent Mr values of 15K, 24K, 43K, 50K, 120K and 160K were detected. An additional polypeptide of 10K was detected by Western blot analysis. The two highest Mr species sometimes appeared as one band, of which only the 120K polypeptide was glycosylated. In the presence of tunicamycin gp120 was no longer detectable and a non-glycosylated precursor of 75K was found instead. Pulse-chase experiments suggested that the smaller polypeptides p24 and p15 are cleavage products of both p160 and p50. Western blot analysis using a rabbit serum directed against p26 of equine infectious anaemia virus (EIAV) and an anti-EIAV horse serum from a field case of infection revealed a cross-reactivity with p24 of FIV. Cat sera collected late after experimental FIV infection recognized p26 of EIAV, indicating a reciprocal cross-reactivity.