The idiotypic patterns detected in the anti-Salmonella typhi serum of one given bleeding have been sought in other bleedings of the same rabbit. Idiotypic patterns carried by antibodies of the second bleeding are not always found in those of the first bleeding. Bifurcated precipitation zones in gels (double diffusion in cells) have been observed in the reaction of several anti-idiotypic sera with the sera of the first and second bleedings of three rabbits, and apparently indicate that idiotypic patterns which were carried by the same molecule in the first bleeding were carried by two separate molecules in the second bleeding. In a comparison of idiotypy of two anti-S. typhi serum samples of one rabbit, the collection of which had been separated by a very long interruption of the immunization, all the idiotypes detected in the early bleeding were also detected in the late bleeding; several idiotypes were detected in the serum of the late bleeding and not in that of the early bleeding. Idiotypy has been observed and compared in IgM and in IgG antibodies. There are, in each of these two classes, antibodies that are not distinguished from antibodies of the other class by anti-idiotypic sera, and consequently that carry the same idiotypic patterns. A comparison has been made between the idiotypy of the two kinds of antibodies which, in anti-S. typhi sera, are precipitated and not precipitated by the polysaccharide. Certain idiotypic patterns are carried only by the antibodies of the first kind. There are also idiotypic patterns which are carried by antibodies of these two kinds, among which certain anti-idiotypic antibodies do not discriminate. Possible cellular implications of observations made on idiotypy at different stages of the immunization of one individual have been discussed.