The adjuvant properties of Micropolyspora faeni, an important source of antigenic material in the production of farmer's lung, were evaluated by comparing antibody- and cell-mediated immune responses of rabbits to bovine serum albumin (BSA) incorporated in complete Freund's adjuvant (CFA), incomplete Freund's adjuvant (IFA) and incomplete Freund's adjuvant with 5-10 mg/ml homogenized M. faeni (MFA). Rabbits immunized with BSA in CFA or MFA developed significantly increased antigen-induced macrophage migration inhibition, lymphocyte stimulation, and delayed skin reactivity when compared to those immunized with BSA in IFA. No similar adjuvant effect on specific antibody production was observed in rabbits immunized using BSA in MFA. These data suggest that M. faeni can act as a selective immunologic adjuvant for delayed hypersensitivity. This adjuvant property might be important in the induction of mononuclear cell infiltrates seen in human hypersensitivity pneumonitis.