Precipitative mixing experiments have been conducted with the aid of a turbidimeter and a darkfield microscope. The results of these experiments, correlated with observations of macroscopic features, indicate that serological precipitation is largely governed by a highly specific mechanism except in the terminal period, when the operation of non-specific forces becomes apparent. The findings are used as the basis for a new description of precipitation in terms of the formation and aggregation of elementary particles of specific precipitate, called seromicrons.