Changes of the in vitro morphogenetic state may be achieved for some potato genotypes, but others are unresponsive (recalcitrant). Although the biochemical basis for somatic recalcitrance is unknown, evidence suggests that two different aspects of oxidative stress may be involved. Phenolic oxidation is a major problem in manipulating cultures of woody plant species (Thorpe & Harry 1990) and lipid peroxidation has been associated with recalcitrance in monocotyledonous plants (Cutler et al. 1989; Benson et al. 1992). Both oxidative phenomena are believed to be free-radical mediated, but to date there is no reported direct evidence for the formation of free radicals during plant tissue culture callogenesis. The objectives of the present study were twofold; to assess the feasibility of using electron paramagnetic resonance (EPR) spectroscopy to detect free radicals directly in plant tissue cultures and to investigate free radical activity during dedifferentiation of responsive and unresponsive potato genotypes.