Serum calcium, inorganic phosphorus and alkaline phosphatase (SAP) have been determined in 75 lactating women and 25 controls. 25 lactating women were re-investigated after administration of calcium and vitamin D supplements for 1 month. Lactating women showed statistically significant hypocalcaemia and raised SAP. However, the elevation of the enzyme level was significantly lower in women with milk intakes of more than ½ litre per day and in those in later months of lactation. The effects of calcium and vitamin D supplementation supported a diagnosis of osteomalacia in a large percentage of lactating women