Fifty breastfeeding mothers were studied at 4, 5, or 6 weeks postpartum. Morning hind milk, serum samples, and stress and infection data were collected to examine whether milk secretory immunoglobulin A (SIgA) is related to demographic, stress, immune, and infection factors. The authors investigated relationships between milk SIgA and maternal stress, mood, symptoms of infection, serum proinflammatory and proimmune cytokines, and cortisol. Older women had lower SIgA, and smokers and lower-income women had higher SIgA. There was a relationship between postpartum infection symptoms and SIgA concentration. The Profile of Mood States-anger score was correlated with higher SIgA and perceived stress with lower SIgA. Positive life events were correlated with higher SIgA. In regression analysis, age, postpartum infections, and serum interferon (IFN)-gamma were significant predictors of milk SIgA. The data suggest that milk SIgA is primarily affected by maternal immune/infection status, but several other stress and mood variables may potentially influence the concentration of milk SIgA.