SUMMARY: The biochemical and serological characters of 36 unclassified cultures are described, and according to these characters the organisms belong to the genera Citrobacter, Aerobacter and Hafnia. While studying the antigenic relationships of the O antigens the known antigenic relationship between some serotypes of Citrobacter and Salmonella was confirmed; new antigenic relationships with S. locarno, S. kentucky, S. uccle and S. tranoroa are described. The antigenic relationships of the Hafnia cultures with Shigella flexneri 4a and with two cultures with a provisional O group of Citrobacter were confirmed.