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Fractionation of the precipitating components of canine prostatic fluids was carried out by gel filtration on Sephadex G-100. By gel filtration three fractions were obtained which were referred to as A, S, and F. Fractions S and F were further purified by recycling through a column of Sephadex G-100. The partially purified components, which were designated S1 and F2, appeared antigenically similar on the basis of gel diffusion precipitation experiments. Further characterization of these components was carried out in order to explain the asymmetric shortening of the precipitin line of fraction S1 and the complete dissolution of the line of fraction S1 when high concentrations of fraction F2 were used. Quantitative precipitation studies, with the fractions and an anti-canine prostatic fiuid serum, showed an unusually narrow zone of equivalence for fraction F2. Fraction S1, on the other hand, gave a much broader equivalence region. The R values for fractions S1 and F2 were found to be 8·5 and 15·0, respectively. Proteolytic determinations showed the presence of caseinolytic activities in both fractions S1 and F2; the activities per mg of these fractions were of the same magnitude. In view of the available immunological data it is suggested that the dissolution of the precipitin lines is due to the creation of a zone of antigen excess provided by high concentrations of fraction F2. [ABSTRACT FROM AUTHOR]