The magnitude and consistency of response to antibacterial agents varies with stage of the production cycle and with the environmental conditions to which the animals are exposed. The level and the spectrum of the drug used contributes to the response; the drugs more effective as therapeutic agents are also more effective as feed additives. A major part of the variation in response among experiments can be attributed to variations in environmental conditions, the initial health status of the animals and the inability to account for normal variation in growth rate and feed conversion with the minimal number of replications used in most experiments. Tissue residues and microbial resistance are the main concerns for human health safety when using antibacterial drugs, the most pressing concerns currently being with the sulfa drugs for tissue residues and with tetracycline and penicillin for microbial resistance. The effects of feed additive usage of antibacterials on efficacy of antibiotics in human medicine are difficult to assess. In animals, the documented evidence supports the thesis that the benefits of this usage outweigh any loss of effectiveness for animal drug therapy. At present, proposals have been submitted in the United States to discontinue the use of penicillin and tetracyclines as feed additives, reserving them for prescribed prophylactic and therapeutic veterinarian use.