In a series of 410 consecutive potentially contaminated operations, we have compared the relative efficacy of intra-incisional cephaloridine and ampicillin in single doses of 1 g in 2 ml of water. In high risk operations (ileocolorectal surgery, perforated appendicitis, or when the patient has 2-5 cm or more of subcutaneous fat at the site of the incision) cephaloridine prophylaxis resulted in a primary sepsis rate of 14 per cent compared with 36 per cent for ampicillin. In all other patients the two antibiotics were equally effective. Secondary sepsis, following discharge from the wound of a substance other than pus, was equally common in the two groups.