Serum, urinary and leukocyte sialic acid content was determined in five bonnet monkeys (Macaca radiata) during the later two-thirds or pregnancy. There was a progressive increase in the levels of serum sialic acid till about one week prior to delivery, followed by a significant decline at parturition and a sharp peak shortly after delivery. The levels came down to non-pregnancy values by two months postpartum. The urinary sialic acid levels were also elevated during pregnancy, but there was no parallelism between the trends exhibited in serum and urine. The sialic acid content of leukocytes was in the non-pregnancy range throughout the period of the study. The results are discussed in relation to the possible role of high levels of serum-bound sialic acid in the protection of the antigenically foreign fetus from the adverse reactions of the maternal immune response.