Of 4434 hospitalized medical patients who received ampicillin in the absence of allopurinol, 251 (5.9 per cent) developed a rash within 21 days of exposure. Of 252 patients who received ampicillin and allopurinol together, 35 (13.9 percent) developed a rash. The present data confirm the previously reported increase in rash frequency in patients who receive both ampicillin and allopurinol.