The protein exoproducts released during exponential growth of Gram-negative bacteria were analysed and compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-Page). The following bacterial strains were tested: Escherichia coli, Klebsiella pneumoniae, Citrobacter freundii, Enterobacter cloacae, Serratia liquefaciens, Serratia rubidaea, Proteus mirabilis, Proteus vulgaris, Salmonella minnesota, Pseudomonas aeruginosa and Pseudomonas fluorescens. It is demonstrated by SDS-Page that members of one species show identical protein pattern, whereas different species show besides comparable protein bands a species characteristic pattern. All members of Enterobacteriaceae were shown to release proteins whose molecular weights fell into the following size regions: Each strain was shown to synthesize a polypeptide of molecular weight 34,000 and one or more polypeptides within the molecular weight range 25,000-29,000. This profile was shown to be clearly different from that of Pseudomonas strains where 20 or more distinct polypeptides ranging from 12,500 to 160,000 Mr were detectable.