The performance of MacConkey broth and minerals modified glutamate lactose medium was examined in a series of experiments. It was found that the addition of seawater with or without antibacterial activity and the addition of 3·2 NaCl solution to double strength media all had similar harmful effects on their performance. Also a concentration of 3·2 NaCl in ordinary lactose broth caused a great decrease (P=0·02) in the number of tubes containing coliform organisms. It is concluded that the high salt content of seawater interferes with lactose fermentation by coliforms. This interference was found to be so great that the number of tubes in which coliform organisms andEscherichia coliwere detected, dropped from 2·5 to 3·5 times when an equal volume of seawater was added to double strength MacConkey and glutamate media. The results of the last experiment suggest that for best performance of the media, the volume ratio of seawater to medium should be equal or less than 1/10. Also glutamate medium was superior to MacConkey broth (P<0·001), especially in the detection ofE.
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