The paper reports evidence of an immunological relationship between one variety of Saccharomyces ceremsise and the Type II variety of Diplococcus pneumonix (Pneumococcus). The most convincing data consisted of the reactions of the Type II bacteria with potent antiyeast serum which agglutinated, and protected mice against these pneumococci as well as the average antiserum obtained by immunization of rabbits with Type II bacteria themselves. The reactivity of the antiyeast serum is strictly specific to the Type II variety of Pneumococcus in the sense that it is entirely devoid of antibodies reactive with Type I or III. The results of absorption experiments with both the antiyeast (rabbit) serum and the anti-Type II (horse) serum were the same as those usually obtained in analogous experiments with immunologically related, but not identical, kinds of bacteria.
The immunological relationship of the yeast and the Type II pneumococcus is apparently based upon S-anti-S reactions. It represents an example of heterogenetic specificity which is of particular interest because of the wide genetic separation of the pathogenic schizomycete and the saprophytic ascomycete.
Data on the individual irregularity in the yeast-agglutinating capacity of serum from non-immunized or "normal" rabbits are presented as experimental facts.
