Our previous studies have defined a highly specific competitive inhibition between a pair of structurally related antigens (GT and GAT) for antigen presentation by accessory cells. The present report investigates this phenomenon in a second antigenic system, which is controlled by a distinct Ir gene product. Two GL phi-specific, I-Ed-restricted, interleukin 2-producing T cell hybridomas were constructed. The antigenic fine specificity of these two hybrid clones was distinct. One hybrid reacted solely with GL phi while the second cross-reacted with GLleu and GLT. These latter two copolymers, as well as the antigen GL, were found to inhibit the GL phi response of the non-cross-reactive hybrid. The structurally related antigen G phi was not inhibitory for this clone's response. The cross-reactive GL phi hybrid could also be inhibited, but, in this case, G phi and not GL caused the inhibition. Reciprocal inhibitions could be demonstrated between these and other hybrids (e.g., GAT responsive), indicating a very high degree of specificity to the inhibition. The inhibition caused by the various copolymers was reversible by increasing the concentration of GL phi, This effect was localized to the antigen-presenting cell and not the T cell hybridoma. Functionally, this competition did not appear to be for antigen uptake or general antigen processing. These findings generalize the phenomenon of antigen competition to a second antigen system in the context of a second Ia molecule. The possible mechanisms accounting for the complex pattern of specificities in this system are discussed.