The role of T and B lymphocytes in rejection of cardiac allografts bearing isolated major histocompatibility complex (MHC) subregion RT1.B/D encoded class II "Ia" disparities was investigated using natural and congenic recombinant inbred rats (RP, PVG RT1r1). RT1.B/D-disparate heart grafts were shown in 3 strain combinations (RP leads to WF, WF leads to [RP x PVG RT1l] F1, and PVG RT1r1 leads to PVG RT1 alpha) to induce a brisk humoral response (complement-dependent cytotoxicity [CDC] titers greater than 2(6] with specificity for class II determinants as evidenced by strain and tissue distribution of susceptible targets. Cytotoxic T cells (Tc) generated in parallel effected equivalent lysis (in the 4-hr 51Cr-release LMC assay) of con A and lipopolysaccharide (LPS) blasts of the same strains. Specificity of Tc for class II determinants was demonstrable by specific inhibition of killing (congruent to 50%) by anti-Ia antibody preparations (monoclonal 0X4 and antiserum ATH anti-ATL) with specificity for class II products of MHC subregions RT1.B and RT1.D, respectively. Adoptive transfer studies revealed that antibody and cytotoxic effectors (W3/25+, OX8+) with specificity for class II "Ia" determinants, though potentially injurious, are not required for rejection of cardiac allografts transplanted across isolated RT1.B/D disparities (RP leads to WF) because transfer even of small numbers (0.5 X 10(6] of helper T cells (W3/25+, OX8-) previously shown to be responsible for adoptive transfer of delayed type hypersensitivity to alloantigen, effected rejection in the absence of detectable antibody or Tc. In the immune response to alloantigen both cytotoxic T cell generation and a humoral immune response develop in parallel with that other form of cellular immunity, akin to classic delayed or tuberculinlike hypersensitivity, itself potentially a potent effector mechanism in organ allograft rejection.