This paper reports a seasonal variation in alkaline phosphatase (ALP) levels in the serum, with low values occurring in summer and high in winter. These variations were found both in indoor and outdoor workers and in a group of elderly women in a long-stay ward. The seasonal change in ALP was in the opposite direction to the seasonal change in the serum levels of 25-hydroxyvitamin D. No such change was seen in the serum levels of gamma- glutamyltransferase. It is suggested that the seasonal change in plasma ALP could be due to changes in the bone isoenzyme. Serum levels of calcium remained unchanged; the change in ALP may reflect a homeostatic mechanism controlling plasma calcium, which compensates for seasonal variations in vitamin D supply.