Five experiments were conducted to evaluate the effects of diet and antimicrobials on weight gain, feed efficiency, ileal weight, and Clostridium perfringens in the ileum of broiler chicks. In the first experiment, glucose, sucrose, and fructose were added to a semipurified diet and the results were compared with those from a practical corn and soybean meal diet. All of the diets were fed with and without bacitracin at a level of 55 ppm. Fructose resulted in the greatest depression in weight gain, followed by sucrose. Bacitracin significantly improved weight gain and feed efficiency of chicks fed the fructose, sucrose, and practical diets. Highly significant inverse correlations were obtained between ileal weight and weight gain and the number of C. perfringens in the ileum and weight gain. In other experiments bacitracin, penicillin, chlortetracycline, oxytetracycline, erythromycin, tylosin, virginiamycin, lincomycin, bambermycins, and carbadox, all at a level of 55 ppm, improved weight gain and feed efficiency and significantly reduced the weight of the ileum and the number of C. perfringens in the ileum of chicks fed the practical diet. The antibacterial agents 3-nitro-4-hydroxy-phenylarsonic acid, arsanilic acid, furazolidone, and sulfathiazole had little to no effect on the 4 parameters evaluated. Virginiamycin and lincomycin at 16.5 and 4.4 ppm, respectively, were shown to be effective. In vitro activities of the antimicrobials against C. perfringens did not directly relate to in vivo activities and the effects on growth and feed efficiency. The results of these experiments support the concept of antimicrobials as growth permittants and provide further evidence for C. perfringens as a causative bacteria for growth depression.