Staphylococcus saprophyticuswas found to differ fromStaphylococcus epidermidisandStaphylococcus aureusby its ability to agglutinate sheep erythrocytes. On testing 30 strains of each species, 28 strains ofS. saprophyticusand one strain each of the other two species, caused agglutination. Twenty‐eight of 30 strains ofStaphylococcus cohniiandStaphylococcus xylosisfailed to cause haemagglutination. The haemagglutinating activity ofS. saprophyticus.when using a 10 per cent bacterial suspension was demonstrated in dilutions of 1:2–1:32. It was reduced twofold, at most, when exposing the bacteria to 56° C for 30 minutes, while no agglutination could be demonstrated after treatment for 10 minutes at 8° C. No haemagglutination could be demonstrated after treatment of the bacteria with a 5 per cent solution of trypsin. Treatment ofS. saprophyticuswith 0.1 M EDTA did not affect the haemagglutinating activity, whereas exposure of the bacteria to 10 per cent trichloroacetic acid reduced the activity. The haemagglutination was D‐mannose‐resistant, and it was inhibited by homologous rabbit antiserum. The agglutinates dispersed when heated at 45–56°C for 30 minutes. A few of the strains ofS. saprophyticustested also agglutinated human, bovine, and guinea pig