A herd of 650 Holstein cows was examined for skin disease. Approximately 400 of the lactating adults were affected, but heifers, calves, and nonlactating cows were clinically normal. The condition was characteristic of primary photosensitization. Milk production of the affected cows was normal. Affected cows did not appear to be ill, and none of the cows was icteric. Three of 7 cows had high serum gamma-glutamyltransferase activities, but in the other 4 cows, activity was within the reference range. Serum activities of other hepatic enzymes were within reference ranges in the 7 cows that were examined. Hepatic biopsy specimens from 3 cows were normal. Specimens from 4 other cows had changes that ranged from minimal to mild, chronic, lymphoplasmacytic periportal hepatitis to acute, random, necrotizing hepatitis. Development of photosensitivity was related to ingestion of alfalfa silage. Acetone extracts of the alfalfa silage, but not of other feedstuffs, were found to inhibit growth of Candida albicans under ultraviolet light. Cows experimentally fed a diet composed exclusively of the alfalfa silage developed skin lesions after 6 days, but did not have detectable serum concentrations of phylloerythrin.