Data are presented on precipitation and C′ fixation in the homologous and cross-reactions of the chicken- and duck-ovalbumin immune systems. The course of specific precipitation with either homologous or heterologous ovalbumin may be described by the same form of equation except that in the reaction between Eac and anti-Ead an alternate form of the equation fits the data more closely. As in the pneumococcal Type III-Type VIII systems, the reaction-curves for the fixation of C′ in the homologous and reciprocal cross-reactions closely parallel those of specific precipitation. Marked similarity in the antigenic composition of the two egg-albumins is indicated both by the precipitin- and C′ fixation-data. At 0 C peak-fixation values in the cross-reaction are almost as high as in the homologous reaction with equal quantities of antibody, the difference being more pronounced in the reaction between Eac and anti-Ead than in the reciprocal reaction.