As a result of chance observations, the authors studied for deoxyribonuclease activity 16 strains of Salmonella, including six fresh isolates and ten stock cultures, with positive results in 13. Reactions characteristically occurred at 48 hours or later, with the majority being manifest at 72 hours and the latest at six days. No other positive reaction was observed among approximately 7,000 Enterobacteriaceae tested, except for Serratia species and Proteus vulgaris. Deoxyribonuclease screening plates for Serratia should be read at 24 hours, and positive results encountered after that time should be interpreted with caution to ensure that H2S-negative strains of Salmonella will not be confused with other genera of Enterobacteriaceae.