Peroxidase activity was measured at three stages, from seedling to maturity, in leaf and stem tissue of two genotypes of Linum usitatissimum. The plants were grown throughout in growth chambers, allowing close control of the environmental conditions. There were large and consistent differences between activities in the genotypes, between leaf and stem tissue, and between dialyzed and undialyzed extracts. Dialysis appeared to remove inhibitor(s). The consistency of the genotypic differences under these conditions permitted reliable comparisons, from the seedling stage to maturity, to be made, a finding relevant to inheritance studies of peroxidase activity.