Potato tubers (Solanum tuberosum) of genotypes that vary in resistance to dark pigment formation when damaged, characteristic of the physiological disorder blackspot, were assayed for free tyrosine. The tubers were also assayed for relative levels of chorismate mutase and proteinase activities, which can regulate free tyrosine levels. The susceptibility of potato tubers to blackspot was shown to be correlated to the amount of free tyrosine by third order regression (R = 0.88). Tyrosine was found to be a limiting factor in pigment development. Chorismate mutase activity (CMI and CMII) was not correlated to blackspot susceptibility of the genotypes studied. Proteinase activities of Atlantic, TXA 763-5, Ranger Russet, Russet Burbank, and Lemhi Russet tuber protein extracts measured with synthetic substrates correlated with blackspot susceptibility. This suggests that the high free tyrosine levels associated with blackspot susceptibility may be due to high levels of proteinase activity in the tuber, rather than tyrosine synthesis.