When Escherichia coli is cultured in nutrient broth at 1 x 10(3) cells/ml, 7.5 h are needed to detect cell growth as turbidity. The time to detect E. coli in this medium is reduced using the alamar blue method. Alamar blue is an oxidation-reduction indicator that changes color in response to cell growth. E. coli can grow in nutrient broth containing red blood cells, but the detection of E. coli based on turbidity is difficult because the red blood cells muddy the medium. As the change in color in the alamar blue method is not affected by red blood cells, the growth of E. coli contaminating red blood cells is detectable. These results suggest that a cell growth indicator such as alamar blue is useful for the detection of bacteria contaminating blood for transfusion.