Monoclonal antibody 5E2 identifies a new rabbit thymocyte specific cell surface molecule designated R-Ta. SDS-PAGE of molecules immunoprecipitated by 5E2 shows that R-Ta exists as a non-covalently associated hetero-dimer consisting of a light polypeptide chain (mol. wt approximately 12,000) and a bi-molecular species of a heavy chain (mol. wts of 45,000 and 40,000). The difference between the two forms of heavy chain can be attributed to different degrees of glycosylation. Each form of the R-Ta heavy chain has a polypeptide mol. wt of 34,000. At least three N-linked oligosaccharides and no significant O-linked sugars were found associated with R-Ta. Two dimensional electrophoresis of V8 protease peptide maps also indicate that the two forms of the heavy chains are similar, if not identical, in polypeptide primary structure. The light polypeptide was found to be serologically and structurally identical to beta-2-microglobulin. This was demonstrated in a previous study by reaction with goat anti-beta-2-microglobulin antisera. In this investigation the structural identity with beta-2-microglobulin was demonstrated by partial amino terminal sequence analysis. The partial amino acid sequence for 18 steps of the R-Ta heavy chain was also determined. A comparison of the amino acid sequence with other known sequences for the conventional Class I molecules of man, mouse and rabbit did not reveal any homology. Thus R-Ta is a new T-cell surface protein, and like human CD1, carries the unique distinction of thymocyte specificity, is beta-2-microglobulin associated, but is not Class I related.