Altho starters made with selected cultures have been used for many years, their satisfactory preparation in quantities still remains one of the big problems of the butter-making industry. Data recently reported have proven that starters contain at least two organisms instead of only the common acid producing type, Streptococcus lactis, and while this information has shown that it is necessary to keep a proper balance between the organisms and has indicated the reason for some of the unsatisfactory results in the carrying of starters, it has not provided methods that enable the butter manufacturer to prepare uniformly good starters. The results herein reported were obtained to determine the influence of such factors as air supply, temperature of incubation, and rate of ripening on the quality of starters.