A total of 1156 Escherichia coli strains including 489 faecal, 384 urinary, 283 other extraintestinal isolates was compared for haemolysin production (Hly), mannose resistant haemagglutinating activity (MRA), presence of antigens K1 and K5 and colicinogenicity (Col). K1 capsule which was demonstrated only in a few serogroups (O1, O2, O7, O18) occurred more frequently among extraintestinal (32.1%) than among faecal (4.3%) or urinary (7.3%) isolates. In the incidence of antigen K5 there was no difference between faecal and urinary (3.3%; 3.1%) or between urinary and other extraintestinal (5.3%) isolates belonging mainly to serogroups O2, O6, O18 and O75. Col+ isolates occurred frequently in all samples (23.5% of faecal, 31.7% of urinary and 43.4% of other extraintestinal strains), they being significantly more frequent in serogroups O1, O2, O7, O18 than in others. A close association existed between K1+ and Col+ properties, mainly (24.4%) among strains isolated from extraintestinal sources other than urine. The frequent coexistence of K1+ and Col+ in serogroups O1, O2, O7, O18 offers a further explanation for the extraintestinal pathogenicity of these serogroups. Neither Hly+ and K1+, nor Hly+ and Col+ were associated. MRA+ and K1+ correlated mainly in serogroups O1, O2 but never occurred simultaneously in serogroup O18. Connection between MRA+ and Hly+ was not associated with other virulence factors (K1, Col). The results showing a close connection among certain serogroups (O1, O2, O4, O6, O7, O18) and certain markers of pathogenicity (MRA, Hly, K1, Col) support the concept that E. coli strains have a clonal connection.