Abstract: Reaction of the peanut explants cultured in vitro to crude R.solanacearum toxin was studied.The results showed that the plant's resistance to the toxin could be determined by either smearing the toxin on the plant or mixing the toxin in the medium.It appeared that the percentage of callus formation on peanut leaf decreased as the toxin concentration increased,though not statistically significant among the samples.Based on the appearance of brown callus,the callus from the susceptible genotypes were more sensitive,showing greater browning,collapsing and other symptoms of damage,at earlier stage after treatment than the resistant plants.The in vitro observations seemed to parallel those in the field.