A hemagglutination (HA) typing system has been developed for demonstrating and characterizing the mannose-sensitive and mannose-resistant hemagglutinins produced by Escherichia coli isolated from human sources. HA typing is performed by testing CFA agar-grown E. coli cells for HA with human, bovine, adult chicken, African Green monkey, and guinea pig erythrocytes in the presence and absence of mannose. Seven major HA types, designated HA type I through HA type VII, have been defined according to the HA patterns produced by 1,334 test cultures consisting of 33 colonization factor antigen I (CFA/I)-positive enterotoxigenic E. coli (ETEC), 37 CFA/II-positive ETEC, 614 isolates belonging to the classical enteropathogenic E. coli, or EPEC, serogroups, 446 non-ETEC, non-EPEC stool isolates, and 204 bacteremia-associated E. coli. Facultatively enteropathogenic E. coli (FEEC) serogroups, which are the causative agents of extraintestinal infections but also sporadic cases of enteritis, comprised 38% of the stool isolates and 91% of the blood isolates examined. Previous observations concerning the HA patterns of CFA-positive ETEC and the EPEC were confirmed. A significant correlation was found between FEEC serogroups and the production of mannose-resistant HA with human, monkey, and usually chicken erythrocytes (the HA patterns designated HA type VI). A large majority (80.2%) of the FEEC strains belonging to the most frequently isolated serogroups from cases of bacteremia (O1, O2, O4, O6, O7, and O18) produced type VI HA patterns. Stool isolates belonging to these same serogroups were 59.2% positive for HA type VI patterns. In contrast, only 17.4% of the non-FEEC stool isolates and 1.9% of the EPEC isolates belong to HA type VI. Of the blood isolates, the HA type VI phenotype was two times more prevalent among K1-positive E. coli than among K1-negative E. coli, 70.6 versus 31.1%. These results suggest that surface-associated hemagglutinins of E. coli, many of which are known to be fimbriae, should be considered in addition to serotype (O:K:H antigenicity) in the description of isolates.