Serum samples from about 10 males and 10 females from each of 16 genetically defined strains of rabbits and from one hybrid were tested as sources of complement for the microtiter lymphocytotoxicity test. Lymph nodes from mice of strain B10.A/Sn were used as the target cells with a strong H-2 alloantiserum. Results indicate marked strain differences. Rabbit serum from a hybrid between the two best strains IIIC/J and IIIVO/J was almost as satisfactory as the parental strains and better than all other strains tested in this system, indicating a genetic basis for the response. The results of testing aliquots of these serum samples against other test systems including human cell lines will have to be in hand before a general statement can be made, but preliminary unpublished data against lymph nodes from A/HeJ and C3H/HeJ strain mice and tumor cells from an ascites form of a sarcoma Sal-A are encouraging and suggest a consistent superiority of the two strains and the hybrid between them referred to above.