Escherichia coli strains isolated from human blood obtained from 539 different patients all over Denmark were examined serologically for O and H antigens. 425 strains could be O grouped with O sera 1 to 150. 90 strains were spontaneously agglutinable. Using O sera to the ten most frequent O groups: 2, 4, 6, 75, 9, 8, 18, 7, 22 and 1, 57 per cent of all strains could be grouped. Using sera corresponding to the ten most frequent O and the ten most frequent H sera, it was possible to O:H type 68 per cent of all O groupable strains. The K1 antigen was detected in 18 per cent of the strains.