Cultures of 10-day-old chick embryo fibroblasts were examined for possible side effects in the presence of amphotericin B-deoxycholate and nystatin. It was found that the recommended dose of amphotericin B-deoxycholate reduced the cell number, altered the transport of small molecules, and decreased DNA synthesis. These effects could be increased by larger doses of amphotericin B. There were no comparable side effects in the presence of the recommended dose of nystatin and only minor side effects at higher doses.