1. The B-vitamins in milk from baboons (Papio cynocephalus) at various stages of lactation were measured microbiologically. 2. Mature milk contained, on average (microgram/ml): thiamin 0.18 riboflavin 0.74, nicotinic acid 3.2, folate 0.03, vitamin B6 0.70 vitamin B12 0.002, pantothenic acid 2.63, biotin 0.0065. 3. Colostrum contained much less vitamin B6 and pantothenic acid than mature milk; otherwise, there were only slight changes in composition as lactation progressed. Neither these changes, nor the absolute values, resembled those for human or cow's milk. 4. The average daily secretion of B-vitamins in milk represented less than 10% of the mother's dietary intake; that of folate was less than 0.5% of the mother's dietary intake. 5. Baboon milk was calculated to provide infants with approximately the following quantities of B-vitamins (/d): thiamin 0.06 mg, riboflavin 0.25 mg, nicotinic acid 1.1 mg, folate 10 microgram, vitamin B6 0.25 mg, vitamin B12 0.7 microgram, pantothenic acid 0.9 mg, biotin 2.2 microgram.