Formula-fed, term infants were evaluated in two studies that assessed the influence on stool characteristics of protein source and iron content of the formula. Parental observations of infants' stool characteristics were recorded on standard forms. Green was the primary stool color for infants receiving whey-predominant formula containing iron at 12 mg/L. A low-iron (1.5 mg/L), whey-predominant formula produced yellow stools. A casein-predominant, iron-fortified (12 mg/L) formula produced primarily yellow or brown stools, although many of the infants fed this formula also had green stools. Soft stools were common in infants fed either whey- or casein-predominant formulas. However, more infants receiving whey-predominant formulas passed watery or slimy stools than did infants receiving casein-predominant formulas. We conclude that green stools in the absence of signs of disease should be considered a normal variation of stool color.