Haemolysin production (Hly) and mannose-resistant haemagglutination induction (MRHA) of human (H), bovine (B), and both (HB) erythrocytes was investigated in strains of E. coli isolated from 142 cases of children with diarrhoea aged 0-36 months. Haemolysin production was more frequent in strains isolated from children under 1 year and this characteristic was strongly associated with HB+ haemagglutination (P < 0.005). Isolation frequency of MRHA strains was compared in 53 breastfed and 50 non-breastfed children under 1 year of age. HB+ strains were significantly more frequent in non-breastfed infants (P < 0.025). Severity of diarrhoea evaluated by the number of watery stools per day, was significantly reduced in the breastfed group (P < 0.05). The results suggests that breastfeeding may protect infants against the establishment of HB+ strains which might be acting either as a main pathogen or as an opportunistic strain.