The effect of different concentrations of fetal bovine serum (FBS) on the growth and survival of three insect cell lines—Aedes aegypti, Antheraea eucalypti, and Bombyx mori—was studied in a series of 10 experiments. Cell viability was low and no growth occurred in any of the cell lines when FBS was omitted from the medium. Maximum growth of A. aegypti cells was obtained with 10% FBS. There was no further increase in this growth when FBS was increased to 20 and 30%, and neither did the increased concentrations have any appreciable adverse effect on the growth or survival of these cells. Maximum growth of A. eucalypti and B. mori cells was obtained in 5% FBS; the growth of these cells was significantly less in 20 and 30% FBS. Viability of A. eucalypti cells was quite low in 20 and 30% FBS. There was, however, no adverse effect on the viability of B. mori cells at these high concentrations.