We have investigated the distribution of the various core types (R1, R2, R3, R4 and K-12) in 138 Escherichia coli isolates obtained from positive blood cultures. Rabbit antisera, raised against five rough strains expressing the respective core types, were made monospecific by extensive absorption. The reactivity of the antisera was tested in ELISA with bacterial cells that had been autoclaved for full exposure of core epitopes. One hundred and thirty strains could be typed directly, while eight strains required prior digestion with proteinase K for removal of cross-reactions. Ninety-four of the strains (68%) expressed the R1 type, and 9 (6.5%), 12 (8.7%), 7 (5.1%) and 3 (2.2%) strains expressed the R2, R3, R4 and K-12 core types, respectively. An R1R4 mixed core type, hitherto not yet described, was found in 13 (9.4%) strains. Results obtained with polyclonal antisera were in agreement with those obtained with monoclonal antibodies to the R1, R2 and R3 core types. Core typing may serve as an additional serological marker next to conventional typing of O-, H- and K-antigens.