Using a monoclonal antibody, Lt-4, directed against human T cell leukemia virus type I (HTLV-I) trans-activator (tax1) antigen, we examined the expression of tax1 and related antigens in a variety of T cell lines bearing HTLV-I and related retroviruses, simian T cell leukemia virus type I (STLV-I) and HTLV-II, by immunofluorescence and immunoblot assays. Lt-4 reacted with all HTLV-I-bearing cell lines tested and five out of eight simian cell lines bearing STLV-I, but not with an HTLV-II-bearing cell line. Lt-4 detected 40 kd tax1 antigen molecules in most HTLV-I-bearing cell lines except one cell line that expressed 39 kd tax1 antigen. In the STLV-I-bearing T cell lines, tax1-related antigen molecules detected by Lt-4 were heterogeneous, having molecular weights in the range of 36-41 kd.