The resistance of Salmonellae to drugs has been studied in the Netherlands since 1958. In 1972, 1973, and 1974 respectively, 14241, 13086, and 22927 strains were tested for resistance to ampicillin, chloramphenicol, kanamycin and tetracycline. From 1973 all strains were also tested for resistance to trimethoprim. In the period covered, the yearly incidence of resistance to at least one of the above drugs ranged from 39.2% to 45.6% of all strains obtained from various sources (humans, animals, animal products, sewage, etc.). A new finding in the period 1972 to 1974 was that many multiply resistant strains emerged in S. typhimurium and in S. dublin isolated from calves and cattle. In 1974, 64.4% of all strains of S. typhimurium from these animals appeared to be resistant to ampicillin, tetracycline, chloramphenicol and kanamycin, and 25.5% of those of S. dublin were found to be resistant to chloramphenicol and tetracycline. Of all strains of Salmonellae examined in 1973 and 1974 respectively, 0.15% and 0.22% were resistant to trimethoprim, the main component of the twin-drug cotrimoxazol. Of the 142 strains of S. typhi isolated in 1972 to 1974 two were resistant to tetracycline only, and one was resistant to all four antibiotics. The others had a normal susceptibility pattern.