The enterotoxigenic Escherichia coli strain H-10407 (078-H11) of Evans, possessing a colonization factor antigen (CFA/I) and haemagglutinating in the presence of mannose, was tested as to its ability to cause haemagglutination in the presence of phenothiazines. The phenothiazines (chlorpromazine, thioridazine, prochlorperazine, triethylpiperazine, promethiazine and promazine) were able to inhibit E. coli haemagglutination when added to the complex E. coli-erythrocytes. The most potent inhibitors were prochlorperazine and thioridazine, which inhibited E. coli haemagglutination in 1 min at a concentration of 1.5 mg/ml. The less potent was promethazine that inhibited haemagglutination at 6 mg/ml. Once haemagglutination had occurred the phenothiazines were able to reverse it, to "unhood" the E. coli from the erythrocytes. Prochlorperazine at a concentration of 0.4 mg/ml could reverse haemagglutination after 1 h contact with the E. coli erythrocytes complex. E. coli H-10407 grown in the presence of phenothiazines lost its haemagglutinating activity. The resulting non haemagglutinating E. coli recovered its haemagglutinating activity when recultured in a prochlorperazine free medium.