Sources of variability in hemoglobin concentration in blood were examined in over 600 rhesus infants at the California National Primate Research Center who had complete blood counts (CBCs) conducted at 3–4 months of age. These infants were born and raised in outdoor social housing. Hemoglobin values ranged from 8.5 to 15.3 µg/dl with a mean and standard deviation of 12.2 ± 0.8 µg/dl. As expected, hemoglobin was strongly associated with the number of red blood cells (RBCs). Plasma protein concentration, an indicator of blood volume, was not a predictor. Associations with infant age, weight and sex, infant serum cortisol, dam's reproductive history, and birth year, month and location were evaluated in regression analyses. Cage of origin, maternal age at delivery and infant weight were associated with hemoglobin concentrations. Unexpectedly, serum cortisol, determined at the same time as CBC samples were taken, was the strongest predictor of hemoglobin concentration. The basis, as well as the functional significance, of the variation in infant hemoglobin and its association with serum cortisol in this population of rhesus fed a nutritionally optimized diet and housed under standard conditions is relevant to the development of both nonhuman and human primate infants. Am. J. Primatol. 75:1139–1146, 2013. © 2013 Wiley Periodicals, Inc.