The 14 mAbs representing workshop cluster 1 recognise a 215/300 kDa antigen expressed on a subpopulation of lymphocytes which express low levels of CD5 but are negative for other B and T cell markers defined by workshop antibodies. Separate studies with cDNA probes for bovine CD3 and T cell receptor indicate that these lymphocytes are gamma/delta T cells. It is of note that the different mAbs react with varying proportions of this cell population, suggesting that the antigen undergoes considerable post-translational modification. A further two mAbs, designated workshop cluster 2, react with a 37/47 kDa heterodimeric molecule expressed in a subpopulation of the WC1+ cells and on an additional small population of T lymphocytes. The cell populations recognised by the two mAbs are different although they overlap in some animals. It is suggested that these mAbs may be specific for T cell receptor molecules.