This thesis investigates the effects of neomycin inclusion in milk replacer on health and performance, as well as gut health and function in preweaned male Holstein calves. When investigating the effects of neomycin on health and performance, it was found that neomycin reduced the severity and length of diarrhea experienced by calves, as well as the number of days with diarrhea. However, few benefits were observed in other indications of health, including growth, feed efficiency, and respiratory illness. When investigating the effects of neomycin on gut health and function, minor alterations were observed in gene expression, fermentation, and histological measures of the gut, but little difference was observed in other metrics. Taken together, the findings of these studies suggest that the oral provision of neomycin in calf production provides little benefit and should be used prudently.