Survival curves for conidiospores of Aspergillus flavus and A. terreus suspended in tissue culture medium containing 25% horse serum and different concentration of amphotericin B (as Fungizone) were compared. Dormant conidiospores were relatively insensitive. Conidiospores incubated in the absence of the drug became increasingly susceptible and the relationship between the duration of preincubation and numbers of conidia surviving consequent exposure to antibiotic was exponential. Of the two species, A. flavus was much more sensitive to amphotericin than A. terreus and in both species conidiospores harvested from young (4 d) cultures were much more sensitive than older conidia, harvested after storage for between 3 and 13 weeks. Variations in death rates for conidiospores of different ages and physiological states and the shapes of the survival curves are explained in terms of antibiotic penetration, pregermination events and germination inhibition.