The efficiency of Agrobacterium tumefaciens-mediated gene transfer in rice was improved by air-drying of the suspension-cultured cells. The suspension-cultured rice calli showed less efficient transformation mediated by Agrobacterium than usual calli cultured on the solid medium. However, simple air-drying of the calli suspension-cultured for 10 to 15min increased the transformation frequency 10-fold or more. The suspension cultures were established from the calli derived from mature rice seeds and subcultured every week. The frequency of transformation varied with the period of suspension culture and the size of the callus. The calli suspension-cultured for 4 weeks showed a higher transformation frequency than the control (not suspension-cultured), and the highest frequency was as high as 20%. However, the frequency was dramatically lowered by the extension of suspension-culture period up to 7 weeks or more. The transformation occurred frequently in the callus 1 to 2mm in diameter. Plant regeneration was observed in transformed calli suspension-cultured for 1 to 12 weeks, but plants abnormal in morphology and seed fertility were regenerated from the calli suspension-cultured for 7 weeks. In conclusion, rice calli suspension-cultured for less than 4 weeks were useful as targets of transformation mediated by Agrobacterium.