1. Exposure of nine different strains ofStaphylococcus aureus in broth to heat treatments up to 5 minutes at 121°C. was found not to alter the Gram reaction of the cocci, neither immediately nor after subsequent storage during 14 days at 5°C. 2. When the same strains were inoculated into canned spinach stew, they showed again no difference in Gram reaction when heated to F>2.6, neither in the fresh product nor after subsequent incubation for 10 days at 32°C. 3. A heat treatment lethal toStaphylococcus aureus but not inactivating its enterotoxin has therefore no influence on the stainability of staphylococci. Hence a bacterioscopic examination of such products as suggested byMossel andZwart (1959) may reveal potentially dangerous pre-process development of staphylococci. Exposure of nine different strains ofStaphylococcus aureus in broth to heat treatments up to 5 minutes at 121°C. was found not to alter the Gram reaction of the cocci, neither immediately nor after subsequent storage during 14 days at 5°C. When the same strains were inoculated into canned spinach stew, they showed again no difference in Gram reaction when heated to F>2.6, neither in the fresh product nor after subsequent incubation for 10 days at 32°C. A heat treatment lethal toStaphylococcus aureus but not inactivating its enterotoxin has therefore no influence on the stainability of staphylococci. Hence a bacterioscopic examination of such products as suggested byMossel andZwart (1959) may reveal potentially dangerous pre-process development of staphylococci.