Summary: The precipitate obtained in the agglutination tube consists of protein and lipoid fractions. It is suggested that under the conditions (increased H-ion and salt concentration) prevailing in the agglutination tube a lipin-protein complex formed from substances in the serum is precipitated. Evidence is cited to show that phenol in 0.5 per cent concentration may cause precipitation of certain protein constituents of serum. It is further indicated that, aside from the metabolic changes accompanying egg laying, such factors as muscular exercise or shock may influence the pH and the lipoid content of the serum and consequently cause “cloudy” reactions. While the predisposing causes for the “cloudy” reaction undoubtedly arise in the serum, the phenomenon of precipitation is nevertheless profoundly influenced by the conditions prevailing in the agglutination tube.