The milk composition of women on a typical rural Mexican diet was compared with that secreted by American women, consuming a diet typical of affluent countries. Milk concentrations of free fatty acids, cholesterol, total amino acids, and selected key minerals were analyzed at 4 or 6 months postpartum. The total milk fat concentration was lower in the Otomi (22.7 +/- 6.7 mg/g milk) than in the American women (31.3 +/- 5.4 mg/g milk, p = 0.001). Although the absolute concentration did not differ, cholesterol, expressed in terms of total lipid, was higher in the Otomi milk (3.9 +/- 1.1 vs. 3.1 +/- 0.7 mg/g fat, p = 0.005). Saturated medium-chain (C10:0-C14:0) and unsaturated intermediate-chain fatty acids (C16:1 and C18:2) were higher in the Otomi than in the American milk (p < 0.03). The concentrations of C16:0, C18:0, and C18:1 were significantly lower in Otomi than in American milk. The milk concentrations of protein and nonprotein nitrogen were comparable between the two groups. The concentrations of serine, proline, cystine, methionine, and tryptophan were higher in the Otomi than in the American milk (p < 0.05-0.001). The concentrations of valine and isoleucine were significantly lower in the Otomi milk (p = 0.05). Expressed per gram of milk protein, the cystine, methionine, lysine, and tryptophan concentrations were higher, and the glutamine/glutamate, valine, isoleucine, and arginine levels were lower in the Otomi milk. The concentrations of phosphorus and copper were lower in the Otomi than in the American milk at 4 months postpartum (p = 0.05). These differences in milk fatty acid and amino acid patterns and mineral content are unlikely to affect infant growth, but may have other biological consequences yet to be ascertained.