Previous studies have demonstrated that anti-K/D antibodies obtained by specific absorption on and elution from relevant red blood cells were capable of inducing enhancement of tumour cells in allogeneic hosts. Anti-Ia antibodies were not found enhancing except when alloimmune sera prepared by skin grafts were used. In the present report, anti-H-2K and anti-H-2D or anti-H-2I sera were obtained in mouse recombinant strains, in order to obviate such absorption and elution procedures. It has been shown that SaI-tumour cells (H-2a, i.e. H-2k/d) can be enhanced in allogeneic recipients, CBA (H-2k) or C57BL/Ks (H-2d) by antisera directed towards H-2Dd or H-2Kk. Alloimmune sera directed towards H2i-coded antigens prepared by immunization with spleen cells had no enhancing activity on the same tumour allografts in the CBA-A/J strain combination anti-IC'D,Sd,Gd) but caused a transitory effect in the C57BL/Ks-A/J combination (anti-IA'K,IBk,IJk,IEk).