Non-precipitating antibodies localized in the IgG a, IgG b and IgG c were isolated from serum of horses inoculated with DNP-HGG, with or without anti-DNP precipitins. The methods for isolation and purification of precipitating and non precipitating antibodies are described. The physicochemical characteristics and biological properties of precipitating and non-precipitating antibodies localized in the same immunoglobulin fraction were compared. Only the IgG a and IgG b non precipitating antibodies can mediate PCA in the guinea pig and the mouse.