Cotyledon explants from mature peanut seeds (Arachis hypogaea L.) were optimized to obtain adventitious shoot buds with high frequencies (>90%). Efficient transformation of these cotyledons by using Agrobacterium tumefaciens strain C58 carrying neomycin phosphotransferase II (nptII) and ß-glucuronidase (GUS; uidA), or coat protein gene of the Indian peanut clump virus (IPCVcp) and nptII on binary vectors (pBI121; pROKII:IPCVcp) led to the production of a large percentage (55%) of transgenic plants. Transformed individuals were obtained through selection on medium containing 125 mg l(-1) kanamycin. A large number of independently transformed plants (over 75) were successfully transplanted to the glasshouse. Integration of the transgenes and stable genetic transformants in the progeny were assessed by PCR amplification of 700-bp fragment of nptII and 585-bp of IPCVcp genes, and Southern blot hybridizations in the T1 generation of transgenic plants. Analysis of 35 transgenic plants of T1 generation from the progeny of a single transformation event suggested the segregation of a single copy insert in a 3:1 Mendelian ratio. On an average, 120-150 days were required between the initiation of explant transformation and transfer of rooted plants to the greenhouse. The cotyledon regeneration system proved to be an excellent vehicle for the production of a large number of independently transformed peanut plants. Shoot formation was rapid and prolific, and a large proportion of these shoots developed into fertile plants. The method reported here provides new opportunities for the crop improvement of peanut via genetic transformation.