A combination of colistin with tylosin, antibiotics differing profoundly by their chemical structure but of a complementary antibacterial activity was examined. In the in vitro experiments sensitivity to antibiotics of 102 E. coli isolates and 68 Salmonella spp. isolates was performed. Among E. coli isolates most isolates were resistant to ampicillin (87.2%), apramycin (73.6%) oxytetracycline (71.2%), sulfometoxazole (75.3%) and amoxycillin (43.1%). Among Salmonella spp. the highest number of isolates was resistant to Oxytetracycline (11.7%). It was found that the growth of E. coli and Salmonella isolates was more effectively inhibited by a combination of colistin with tylosin than by colistin alone. This situation is exemplified by a decrease of MIC value of E. coli for colistin + tylosin (range 0.05-0.2 µg/ml) in comparison to the value of MIC for colistin (0.08-0.2 µg/ml). In experimental chicken colibacteriosis both prophylactic and therapeutic use of the combination of colistin with tylosin completely eliminated mortality and lowered morbidity.