A retrospective investigation into the drug resistance pattern of intestinal E. coli was carried out in 17 closely monitored herds in order to evaluate a possible influence of an improved diagnostic and a restrictive use of antibiotics in swine herds. The herds participated from October, 1970, in an investigation of preweaning mortality and morbidity, and during a two-year investigation period antibiotics were only used when therapeutically indicated. Sensitivity tests were regularly performed on the intestinal flora from post mortem examined pigs from the herds and the results forwarded to the local veterinarians. A total of 443 E. Coli strains were isolated in pure or almost pure culture from the jejunum of post mortem examined pigs and tested for sensitivity, using the following antibiotics: streptomycin, compound sulphonamide, neomycin, oxytetracycline, chloramphenicol, nalidixic acid, furazolidone, and ampicillin. During the investigation period, the frequency of isolated multiresistant E. coli strains decreazed from 67.7 per cent to 9.5 per cent and the number of strains without antibiotic resistance increased from 3.0 per cent to 36.2 per cent (Table I; Fig. 1). Herds were divided into 3 groups according to different intensity of antibiotic administration and to closed or open management system (Table II). The most pronounced decrease in the number of multiresistant E. coli strains (from 64.9 to 2.0 per cent) was observed in Group I, which comprises closed herds with a rapid decrease in the use of antibiotics. In Group II, which comprises semi-closed herds with a decreasing, but in the last year of the investigation still frequent use of antibiotics, a somewhat slower decrease of the number of multiresistant strains to a level of 12.1 per cent was observed. The number of strains without resistance increased to approximately the same level as in Group I. In Group III, which comprises open herds with infrequent use of antibiotics, the incidence of resistant E. coli strains fluctuated (Table III; Fig. 2). The resistance was proved transferable in 60.5 per cent of the examined resistant strains. The frequency of transferability did not show any significant change during the investigation period (Table IV). The most frequently encountered resistance determinants were: sulphonamide (97 per cent), streptomycin (40 per cent), and oxytetracycline (34 per cent). Only resistance against sulphonamide, streptomycin, tetracycline, and chloramphenicol was proved transferable (Table V). It is concluded that it is possible to obtain a simultaneous reduction of induced antibiotic resistance of intestinal E. coli and of enteric diseases in sucking pigs in commercial herds, by adjustment of management and rational use of antibiotics based on improved diagnostic including in vitro sensitivity tests.