Ninety-five Escherichia coli isolates from bovine mastitis, 47 isolates from milking machine filters, 36 enterotoxigenic (ETEC) and 43 verocytotoxigenic (VTEC) isolates from cows were examined for the ability to resist the bactericidal effects of 90% gnotobiotic calf serum. There was no significant difference in the percentage of isolates in each group which demonstrated resistance. Two potential virulence traits, the traT gene and the K1 capsular antigen, previously shown to be related to serum resistance, in human E. coli pathogens, were also examined. Using colony blot hybridization there was no significant difference in the percentage of isolates in each group carrying the traT gene. A significant relationship between the presence of the traT gene and serum resistance was not found in any of the four groups of E. coli isolates tested. Only 3.2% of the bovine mastitis, 2.1% of the milk filter and 4.6% of the VTEC isolates were positive for the K1 capsular antigen. Again, no correlation between either the K1 antigen and serum resistance or between the K1 antigen and the presence of the traT gene was found in any of the four groups. None of the antimicrobial resistance patterns of the isolates were the same as those demonstrated by R plasmids known to carry the traT gene. Thus, it appears that the traT gene may not be related to serum resistance in bovine E. coli isolates.