High concentration of ampicillin and the Eagle effect among gram-negative rods.

79 Escherichia coli strains were investigated by the triple layer technique for the presence of the 'paradoxical zone phenomenon' around an ampicillin disc. 76 strains were positive, 3 negative. 7 positive and 3 negative strains were treated with varying concentrations of ampicillin and counted. Only 5 of the 7 positive strains showed the Eagle effect, all 3 negative strains gave no Eagle effect. The magnitude of the Eagle effect observed is regarded as being of no clinical importance in the treatment of E. coli with high dosages of ampicillin.