Calli of Nicotiana tabacum (tobacco) were treated with two dose ranges of aflatoxin B1 (0.1-2.0 μg ml-1 - low does; 5-25 μg ml-1 aflatoxin B1). The ability of calli to recover following 3 weeks of toxin exposure was also investigated. The I50 (50% inhibition) value for fresh mass accumulation was approximately 2 μg ml-1 AFB1. Fresh mass accumulation was significantly lower than the control value from 0.5 μg ml-1 AFB1. Following 3 weeks growth without a toxin source, the growth of calli up to and including 10 μg ml-1 AFB1, was significantly greater than control calli, indicating reversibility of the toxic effects. With increasing toxin concentration, chlorophyll content of callus was inhibited from 0.5 μg ml-1. Transfer to a toxin-free medium resulted in a degree of recovery (up to 0.5 μg ml-1). In the dose range 5-25 μg ml-1, the levels of chlorophyll were drastically reduced, with no recovery following AFB1 removal. Electron microscopy revealed a disruption of chloroplast structure as an early deteriorative event in AFB1 exposure of callus cells. Protein levels were less sensitive, with inhibition manifested only in the high dose range. Shoot development occurred at all concentrations, but was significantly inhibited from 5 μg ml-1 AFB1. Recovery following toxin removal was minimal at these higher AFB1 concentrations. The number of necrotic calli increased progressively from 5 μg ml-1 as toxin levels increased.