The primary structure analysis of the gag gene products of human T-cell leukemia virus (HTLV)-ICR has been nearly completed. A comparison of the amino acid sequences with the published nucleotide sequence of HTLV-IATK established that i) p19 which is known to share antigenic determinants with a protein present in normal thymic epithelium, is nevertheless virally coded. ii) The gene order and complete primary structure of the gag precursor (Pr55) which has been shown to be myristylated (My) at its N-terminus is My-p19-p24-p15-OH; and iii) the Pr55gag amino acid sequences of HTLV-ICR and HTLV-IATK are nearly identical showing only a single residue difference in the C-terminal region of p15. Antibodies to synthetic peptides inferred from the nucleotide sequence of the env gene of HTLV-IATK were also raised and used to identify and purify env precursor gPr62-68, surface glycoprotein gp46-51 and transmembrane protein p21. While most of the peptide sera were shown to be subgroup specific some of them detected antigenic determinants shared between protein homologs of viruses of subgroups I and II. Partial or complete amino acid sequences of both the gag and env gene coded proteins of bovine leukemia virus (BLV) structural proteins have also been determined. These extensive protein data together with nucleotide sequences confirm and extend our initial finding that HTLV and BLV are structurally and antigenically related and may have originated from common ancestor. The structural and immunological studies revealed also relationships between HTLV and a number of type C and type D retroviruses studied. One of the highly conserved sequences is shared by the transmembrane proteins of these retroviruses which have been implicated in immunosuppression. It is conceivable that these common regions have common biological function. Two previously unidentified proteins of BLV have also been purified and structurally characterized. Nucleotide sequences capable of coding for related products are present in HTLV. The nature and possible biological functions of these new BLV proteins and the putative HTLV gene products will be discussed. The size and complexity of the genome of the replication competent retroviruses are similar but not identical. The 35S RNA of all replication competent helper viruses is divided into three genes encoding the viral structural proteins: the gag (group-specific antigen) gene codes for the internal structural proteins, the pol (polymerase) gene codes for the enzymes protease, reverse transcriptase and endonuclease and the env (envelope) gene codes for the proteins of the viral envelope.(ABSTRACT TRUNCATED AT 400 WORDS)