Immunogenicity, eliciting antigenicity of MT-141 and its cross-reactivity with other beta-lactam antibiotics were studied in mice, guinea pigs and rabbits. The results were as follows. Injections of MT-141 failed to produce IgE-type antibody in mice but injections of the MT-141 conjugated to rabbit serum albumin produced a trace of IgE-type antibody. No antibody was produced in the guinea pigs immunized with the MT-141 conjugated to rabbit serum albumin in alum or Freund's complete adjuvant. The conjugated MT-141 also failed to elicit anaphylactic shock in the immunized guinea pigs. The subcutaneous treatments with MT-141 in Freund's complete adjuvant produced an amount of hemagglutination antibody in rabbits. The intravenous treatments with MT-141 produced no antibody in rabbits. When rabbits were subcutaneously immunized with the MT-141 conjugated to rabbit serum albumin in Freund's complete adjuvant, production of specific antibody in the rabbits was demonstrated by observations of passive cutaneous anaphylaxis and hemagglutination. The results of hapten-induced inhibition of passive hemagglutination, passive cutaneous anaphylaxis, anaphylactic shock and hemagglutination by using conjugates of antibiotics and rabbit serum albumin as immunogens and conjugates of antibiotics and bovine gamma-globulin as eliciting antigen showed that MT-141 did not cross-react with other antibiotics. MT-141 did not cause the in vitro direct Coombs' reaction in the human blood even at a high concentration of 160 mg/ml. It is concluded from these results that immunological activity of MT-141 preparation is weak.