The intestinal contents of 17 neonates with necrotizing enterocolitis were analyzed for pH, carbohydrate, protein, and bacteria. The intraluminal pH was less than 5.0 (16/17). Sufficient carbohydrate and bacteria capable of fermenting the carbohydrate to organic acids were found. The intraluminal protein content was greater than 5 g/dl. The variables of acid and protein were then examined in a rabbit intestinal loop model. The hemorrhagic response in individual loops was measured using Cr51 tagged red blood cells such that the microliters of blood per centimeter intestine could be determined. Loops with organic acid and protein had significantly (p less than 0.01) more intramural blood than control loops. Organic acid (possibly generated by bacterial mixed acid fermentation of carbohydrate) in the presence of protein promotes intramural hemorrhage similar to that seen in neonates with necrotizing enterocolitis.