A total of 64 isolates of Escherichia coli secured from cases of intramammary infections were tested for their ability to hemagglutinate bovine erythrocytes; of these 37 (58%) were hemagglutination positive. Only 2 of 12 fecal  Escherichia coli isolates (17%) obtained from healthy controls were hemagglutination positive. humans. This significant association of hemagglutinating Escherichia coli and intramammary infections indicates the likelihood that hemagglutination is a marker of virulence. Only 16% (3 of 19) of Proteus species and 13% (1 of 8) of Klebsiella pneumoniae mastitis isolates were hemagglutination positive. There was a significant correlation (P < 0.025) between hemolysin production and hemagglutination; 67% (16 of 24) of the isolates that produced hemolysin also hemagglutinated bovine erythrocytes. There was no significant correlation between hemagglutination and motility, as there was a trend for flagellated organisms to be non hemagglutinators.