When 5 mg doses of beryllium hydroxide, Be(OH)2, were injected intravenously (i.v.) into rats, much of the injected material was retained in the lungs. When the injection of Be(OH)2 was accompanied by particulate antigens such as killed B. abortus organisms or SRBC, immune responses took place in the intra-thoracic lymph nodes (ITLN) so that antibodies of the IgA class were generated and endowed the bile with significant agglutinating activity. This did not happen when the antigens were injected without the beryllium adjuvant. Similarly, a soluble antigen, bovine serum albumin (BSA) only gave rise to significant amounts of biliary and serum antibodies after i.v. injection if Be(OH)2 was used too. The highest titres occurred when the antigen followed the adjuvant after an interval of a few seconds but substantial titres still resulted if the injection of antigen was delayed for 24 hr. The amounts of biliary antibodies generated by the parenteral injection of antigen and adjuvant were comparable to those that occurred after the same materials were injected directly into the GALT. The adjuvant action of Be(OH)2 was able to induce in athymic (nude) rats the production of measurable amounts of both biliary and serum antibodies to SRBC.