Resting T cells spontaneously adhere in a selective manner to potent accessory cells, such as dendritic cells (DC) and lymphoblastoid B blasts (LCL). Here we demonstrate that leukosialin (CD43) and major histocompatibility complex class I molecules (MHC-I) might play a critical role in this process. T cell conjugate formation with monocyte-derived DC (md-DC) and LCL could be strongly inhibited by either preincubating T cells with Fab fragments of CD43 monoclonal antibody (mAb) 6F5 or by preincubating md-DC or LCL with MHC-I mAb W6/32. Intact CD43 mAb 6F5, in contrast to monovalent Fab fragments, enhanced T cell adhesiveness by transactivating CD2 binding to CD58 molecules. Interestingly, induction of this proadhesive signal via CD43 with intact 6F5 mAb was found to revert mAb W6/32-mediated inhibition of T cell conjugate formation. These observations indicated that CD43 cross-linkage mimics and monovalent mAb 6F5 inhibits interaction of T cell CD43 with a stimulatory ligand on opposing cells, presumably MHC-I. For the demonstration of direct physical interaction between CD43 on T cells and MHC-I-coated beads it was necessary, however, to ligate CD2 on T cells with a stimulatory pair of CD2 mAbs (VIT13 plus TS2/18). This suggests that CD2 ligation crosswise upregulates CD43 binding avidity for MHC-I and that both adhesion molecule pairs (CD43/MHC-I and CD2/CD58) act in concert to induce and mediate T cell conjugate formation with certain cell types.