'Double negative' (CD4-CD8-) thymocytes from adult mice of different inbred strains were examined for surface expression of CD3 and of various forms of the T cell antigen receptor (TcR), as well as for the levels of subpopulations defined by the surface markers HSA ('heat stable antigen', recognized by M1/69, J11d and B2A2), CD5 (Ly 1) and Thy 1. Marked variations were found in the level of the double negative subsets which were surface TcR+, or which were HSA-CD5+; these generally varied together since most CD4-CD8-HSA-CD5+ thymocytes were TCR+. The level of the CD3-TCR complex on the surface of those double negative thymocytes which were TcR+ was as high as on mature T cells in some strains (CBA/Ca), but was much lower in other strains (C57BL/6J). In most mouse strains the CD4-CD8-HSA-CD5+ thymocytes expressed predominantly the alpha beta form of the TcR, with an exceptionally high (70%) usage of V beta 8 gene products. In strains which lacked V beta 8 expressing T cells due to a deletion of the V beta 8 gene region, reduced levels of alpha beta TcR+ cells were found within the CD4-CD8- thymocytes; the HSA-CD5+ subset was then only present at low levels (as in SJL/J and C57BR mice) or was present at a high level but expressed predominantly gamma delta TcR (as in SWR mice). The results suggest that the accumulation of CD4-CD8-TcR+ HSA-CD5+ thymocytes is a selective event, and that their developmental pathway is off the mainstream of T cell maturation in the thymus.