A bidirectional regulatory interaction between the central nervous system and the immune system is largely provided by cytokines and their specific receptors, which are expressed by cells of both systems. Transforming growth factor-β1 (TGF-β1), produced by glial cells and lymphocytes and regulated by steroid hormones, is one such cytokine. In the current study, we examined the relationship between TGF-β1 and peer affiliation in bonnet macaques (Macaca radiata) either reared normally or exposed as infants to conditions in which their mothers faced fluctuating requirements for food procurement (variable foraging demand [VFD]). Rearing under VFD conditions has been previously shown to produce dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis in these animals. Serum levels of TGF-β1 after exposure to a moderate stressor had no correlation with peer affiliation under baseline conditions (r=.07), but were highly correlated with affiliation after subsequent challenge with a fear stimulus (r=.62). Affiliation after the fear stimulus also was inversely correlated with baseline levels of affiliation (r=−.71). These data suggest that changes in peripheral TGF-β1 may be reflective of latent behavioral and biochemical propensities possibly related to affect. Further examination of the effects of early adversity will improve our understanding of the relationship between the HPA axis and immune function.