During the summer of 1975 (from June 30th to July 28th) weekly milk samples were obtained from 49 cows kept on pasture during the daytime, and from 48 cows which were confined indoors in stanchion barns and fed grass twice a day. The cows on pasture were exposed to relatively moderate temperatures, which were somewhat lower than for the indoor group (Table I). In the cows kept on pasture, an increase in the milk somatic cell content (MSCC) was observed throughout the experimental period, whereas no corresponding increase was recorded in the indoor group (Table III). This increase in MSCC appeared to occur primarily in non-infected quarters (Table IV). The data from the present investigation seem to indicate that the seasonal increase in MSCC during the summer, which has been reported by several other investigators, cannot be directly related to environmental temperature per se.