Data were analyzed to delineate the diurnal variation in body cell content of quarter-milk samples. Frequent foremilk samplings indicate that cell counts are lowest just prior to milking, rise sharply in the strippings, remain high during the next 4 hr, then decrease progressively to a minimum value near the end of the intermilking period. The body cell level and the magnitude of cyclic change varied widely between groups of cows, cows within group, and to a lesser degree between quarters of a cow. These results emphasize the necessity for care in selecting the optimum time of sampling according to the type of information desired in research or routine health surveys. Variation in time of sampling can introduce real uncertainty in the interpretation of cell count data.