Sera from 12 patients with farmer's lung disease (FLD) drawn during an acute episode and displaying precipitins to Micropolyspora faeni were analyzed by crossed immunoelectrophoresis with an intermediate gel combined with a rabbit reference precipitate system. Forty-six precipitin arcs were identified in the reference system, and FLD sera reacted with 36 of these antigens. The FLD sera displayed significantly more precipitates and higher precipitin scores when compared with sera from 16 precipitin-positive subjectw who did not have FLD. However, there was overlap between the 2 groups, and neither measurement distinguished the 2 persons who became ill after challenge from one person who did not. Three disease specific antigens were identified that may have special relevance for pathogenesis of disease. Ten exposure-relevant antigens were also identified. We propose that these antigens be important components of a standardized M. faeni antigen preparation.