The antibacterial and bactericidal activities of the antihistamine trimeprazine were studied against 243 strains of bacteria which included both Gram positive and Gram negative types. The susceptibility of these bacterial strains to trimeprazine was assessed by determining their minimum inhibitory concentration (MIC) which was found to be between 10 and 100 μg/ml. Nineteen strains of Staphylococcus spp. were inhibited at 10-50 μg/ml of trimeprazine. Most of the strains belonging to Bacillus spp. and Salmonella spp. were inhibited by less than 100 μg/ml. Trimeprazine could also inhibit strains of Shigella spp. Vibrio choleras and V. parahaemolyticus at 10-100 μg/ml. Strains of klebsiella, proteus, pseudomonas and citrobacter were moderately sensitive to trimeprazine. In in vivo studies it was seen that when trimeprazine was used at a concentration of 0.75 and 0.4 μg/gm body weight of the mouse both levels offered significant protection to Swiss mice challenged with 50LD50 of virulent strain of S. typhimurium 74. Statistical analysis of the data was found to be significant, p<0.001 according to χ2 test. © 1997 Akadémiai Kiadó.