Cryptosporidia that were infective for infant mice were subjected to various moist heat treatments and subsequently tested in infant mice for infectivity. Warming from 9 degrees C to 55 degrees C over a period of 15 to 20 minutes neutralized infectivity in calf feces, cecal contents, and ileal scrapings. Inocula held at 45 degrees C for 5 to 20 minutes lost its infectivity. These results suggest that pasteurization of raw milk, circumstantially incriminated in cryptosporidial transmission, would neutralize cryptosporidial infectivity.