1. Complete lipid patterns of specific precipitates from horse and rabbit Type I antipneumococcus sera, as well as of the sera themselves, have been determined by gasometric micro methods.
2. The lipid patterns of horse and rabbit antisera are very similar, and as regards the phosphatide fractions are relatively identical.
3. The lipid patterns of specific precipitates from horse and rabbit antisera show one outstanding qualitative difference. The specific precipitate from horse antiserum contains an amino phosphatide, which is probably cephalin, while that from rabbit antiserum contains a non-amino phosphatide, which is thought to be lecithin.