The aim of this study was to investigate serum digoxin-like immunoreactive substance (DLIS) levels in 60 healthy term infants when they reached 1-6 months of age with regard to feeding regimen. Group I consisted of 30 infants fed exclusively on breast milk. Groups II and III each consisted of 15 infants fed on formula and cow's milk, and on formula and cow's milk supplemented by breast milk, respectively. Mean serum DLIS concentrations were 0.03 +/- 0.05, 0.18 +/- 0.09 and 0.15 +/- 0.09 ng/ml in groups I-III, respectively. The difference between the DLIS levels in groups II and III was not significant. Serum DLIS levels of infants in groups II and III, on the other hand, were significantly higher than in group I (p < 0.05). These findings were interpreted to suggest that artificial nutrients may cause volume expansion and an increase in endogenous DLIS levels. The latter response is possibly a protective mechanism to decrease volume expansion in groups II and III.