OEP--Original Endotoxin Protein--is a protein moiety of endotoxin of Pseudomonas aeruginosa. An adjuvant action of OEP was investigated in mice immunized with sheep red blood cells (SRBC). Several features characteristic of the adjuvant action of OEP have been revealed; OEP shows no effect on simultaneous treatment with the antigen, and is most effective on successive treatments given after SRBC. The effective dosages of OEP as an adjuvant were in the range of 10 to 100 mug per mouse. Experimental data suggested that OEP might enhance the antibody production in its early phase. The adjuvant action of OEP was lost in mice pretreated with large dosage of OEP; OEP-tolerance could be induced to its adjuvant action. The mechanism of the adjuvant action of OEP was discussed comparing with other adjuvants.