Sera from patients infected with Escherichia coli, Proteus, Klebsiella, and Serratia were studied for precipitins to ultrasonic extracts of these organisms in gel-diffusion plates. Sera from 66 per cent of these patients contained precipitins when initially tested. Twenty-four per cent of sera tested in the first week after onset of infection contained precipitins, but this rose to 78 per cent by the third week. Cross-reactions of sera with Pseudomonas antigens were unusual, but were common with other enterobacterial antigen extracts. However, higher titers were usually present to homologous as compared to heterologus antigens. Sera from seven patients contained precipitins to a common enterobacterial antigen. Precipitins to E. coli, Proteus, Klebsiella, and Serratia were detected in only a small proportion of control sera.