The optimal contition of inducing the primary antibody by incubating sheep red blood cells(SRBC)with human tonsil cell was investigated in vitro. The proper cell density in culture was found to be 0.5—1.0 10~7 nuclear cells/ml. More antibody producing cells(plaque-forming cells,PFC)could be induced by add- ing both calf and horse serum in the culture than by adding only either one of them. Using proper dosage of insulin,lipopolysaccharide(LPS)or thymidine and hypoxan- thine could also increase the number of PFC.The rate of PFC against SRBC induced in the culture was estimated to be about 100～300 PFC/10~6 nuclear tonsil cells.