Three mAb, anti-Ti gamma A, anti-TCR delta 1, and anti-delta TCS1, have been developed against the CD3-associated gamma/delta molecular complex. One of this antibody anti-Ti gamma A is specific for an epitope encoded by the V9 gamma-gene. The two others react with the delta-chain but their fine epitopic specificity has not been characterized previously. In the present study, we have compared the surface expression of these three antigenic determinants on 27 cloned and 5 polyclonal CD3+ TCR gamma/delta + cell lines derived from human peripheral blood of 13 distinct individuals. It was found that all CD3+ TCR alpha/beta- clones and polyclonal cell lines tested were recognized by anti-TCR delta 1. In contrast, only a fraction of both clones and cell lines reacted with either anti-Ti gamma A or anti-delta TCS1 mAb. In fact reactivity of the latter reagents was found to be mutually exclusive on the cell panel. Northern blot analysis of RNA extracted from a series representative clones showed a positive correlation between the surface expression of the delta TCS1 epitope and the transcription of the V-delta gene isolated from the IDP2 cell line. These data support the view that anti-TCR delta 1 can be used to positively define the entire TCR gamma/delta+ fraction. Moreover, the reciprocal reactivity of anti-delta TCS1 and anti-Ti gamma A on cultured cell lines suggests that these reagents should delineate in human peripheral blood distinct, essentially non-overlapping, subsets. Taken together, the present results indicate that the complementary use of these three antibodies will be helpful to further characterize the TCR gamma/delta + peripheral lymphocyte fraction.