A modification of the passive immune hemolysis method for the determination of the production of thermolabile enterotoxins by V. cholerae and E. coli is proposed. This modification permits the use of solid culture media. Experiments with cholera enterotoxin have demonstrated that the sensitivity of the modified method is 8-10 times higher than that of the Elek method. Similar results have been obtained with the use of the proposed method in the study of the capacity of different V. cholerae and E. coli strains for producing enterotoxins. The results obtained with the use of this method have been found to correlate with those obtained by means of the skin test and passive immune hemolysis in a liquid medium. We have used the modified method in the study of the production of thermolabile enterotoxin in transconjugants obtained by the hybridization of E. coli strain GA 107 carrying plasmid pCG86 which determines the synthesis of thermolabile and thermostable enterotoxins and E. coli strain K12 C600 R. The results obtained in the study of toxin formation in 99 transconjugants, carried out with the use of the proposed method, the skin test and passive immune hemolysis, have been shown to coincide.