The changes in the production of antibody against Salmonella typhimurium antigen were investigated in rats by means of the agglutination test after chronic oral administration of the L-asparaginase inhibitors morphine (M) or D-aspartic acid (D-Asp) alone or together with L-aspartic acid (L-Asp) and food restriction, all of which had been started five days before the injections of antigen. The statistical evaluation, carried out after the titers had been defined as -log2 of the highest dilution giving a positive agglutination reaction, showed that M or D-Asp significantly decreased antibody production in comparison with the immunized control or food restricted group. The concomitant administration of L-Asp appeared to significantly antagonize the inhibitory effect of both M and D-Asp. Therefore, the results were considered as further supporting evidence for the fact that the deleterious effect of M on the immune system and its functions might be related to the inhibitory effect of M on L-asparaginase activity.