The immunosuppressive effect of serum from mice heavily infected with BCG(BCG-serum) was evaluated. The delayed-type hypersensitivity reaction to sheep erythrocytes was depressed when BCG-serum was administered both systemically and locally at the same time of challenge. This study shows that serum of animal infected with mycobacteria contains a factor responsible for immunosuppression and factor may be important in understanding the mechanisms of allergy in mycobacteria] infections.