In a prospective, open, uncontrolled study 25 patients infected with the human immunodeficiency virus with chronic refractory diarrhea and either confirmed cryptosporidiosis (n = 7) or absence of demonstrable pathogenic organisms (n = 18) were treated with a daily oral dose of 10 g of an immunoglobulin preparation from bovine colostrum over a period of 10 days. Among the 7 patients with cryptosporidiosis, this treatment led to complete remission in 3 and partial remission in 2. Among the 18 patients with diarrhea and negative stool culture, complete remission of diarrhea was obtained in 7 and partial remission in 4. In the remaining 2 patients with cryptosporidiosis and the 7 patients with diarrhea but no demonstrable pathogens treatment produced no significant improvement of the diarrhea. Subsequent doubling of the Lactobin dose (2 x 10 g daily) in 8 of the nonresponders led to complete remission in one case and at least partial remission in a further 4 patients. Treatment of refractory diarrhea with 10 g immunoglobulins from bovine colostrum per day constitutes an important therapeutic approach and led to complete (40%) or partial (24%) remission of diarrhea in 64% of the patients described here.