The effect of amphotericin B (AB) and amphotericin B methyl ester (AME) on viability and DNA synthesis in three insect cell lines, Trichoplusia ni (Tn) Carpocapsa pomonella 169 (Cp) and Aedes aegypti (Aa), has been evaluated. In all cases AME was less toxic than AB and inhibited DNA synthesis to a lesser degree than AB. However, the three cell lines differed in their response to the two polyene macrolide antibiotic preparations. Tn and Aa cell responded in a similar manner when exposed to either AME or AB, whereas Cp cells were more sensitive to both antibiotics than Tn or Aa cells.