Four patients with lymphoproliferative disease of granular lymphocytes (LDGL) coexpressing CD3 and the natural killer (NK)-related "p58" receptor for HLA-C alleles were studied. These CD3+p58+ LDGLs have been detected among a series of 44 CD3+ LDGLs analyzed. Two patients with LDGL (GI and BA) expressed only the p58 molecule defined by the GL-183 and CH-L monoclonal antibodies (MoAbs), while the cases of patients PU and MA also coexpressed the molecular form identified by EB6 anti-p58 MoAb. Three LDGL cases (GI, MA, and PU) displayed the CD8+4-CD16+ T-cell receptor (TCR)alpha/beta+ phenotype, while one patient (BA) was CD8+4+CD16+ TCRalpha/beta+. Freshly isolated granular lymphocytes (GL) from these cases displayed cytolytic activity in an anti-CD3 MoAb-triggered redirected killing assay against the Fcgamma-receptor+ (Fcgamma-R+) P815 target cell line. Lysis of P815 target cells, triggered by an anti-CD3 or by anti-CD16 MoAb, could be inhibited by the addition of anti-p58 MoAb in three fresh or interleukin (IL)-2-cultured GL tested (GI, MA, and PU). Triggering of cytotoxicity against the HLA-DR+ Fcgamma-R+ Daudi cell line induced by appropriate superantigens could also be inhibited by anti-p58 MoAb in patients PU and GI with LDGL. These data indicate that activation through the CD16, CD3, and TCR molecules can be modulated by p58 receptors in these LDGLs. On the contrary, IL-2-expanded cells of patient BA were induced to lyse P815 target cells by anti-p58 MoAb. In addition, anti-p58 MoAB enhanced anti-CD16 MoAb triggered lysis and did not inhibit activation via CD3. These data indicate that, in this particular patient with LDGL, p58 displays a stimulatory effect on cell triggering, rather than the typical inhibitory effect previously observed in p58+ T-cell clones derived from healthy donors. The anti-p58 MoAb did not induce CA++ mobilization in p58+ LDGLs and in a p58+CD3+ normal T-cell clone equipped with inhibitory p58 molecules, while Ca++ mobilization could be observed in cultured GL from patient BA, which could be activated by anti-58 MoAb. These findings suggest that stimulatory and inhibitory p58 molecules are equipped with different signal transducing properties, thus contributing to a better knowledge of the normal counterpart.