Clonal replicates of different transformed potato plants expressing transgene constructs containing the constitutive Cauliflower Mosaic Virus (CaMV) 35S promoter, and sequences encoding the plant defensive proteins snowdrop lectin (Galanthus nivalis agglutinin; GNA), and bean chitinase (BCH) were propagated in tissue culture. Plants were grown to maturity, at first under controlled environmental conditions, and later in the glasshouse. For a given transgene product, protein accumulation was found to vary between the different lines of clonal replicates (where each line was derived from a single primary transformant plant), as expected. However, variability was also found to exist within each line of clonal replicates, comparable to the variation of mean expression levels observed between the different clonal lines. Levels of GNA, accumulated in different parts of a transgenic potato plant, also showed variation but to a lesser extent than plant-plant variation in expression. With the majority of the clonal lines investigated, accumulation of the transgene product was found to increase as the potato plant developed, with maximum levels found in mature plants. The variation in accumulation of GNA among transgenic plants within a line of clonal replicates was exploited to demonstrate that the enhanced resistance towards larvae of the tomato moth, Lacanobia oleracea L., caused by expression of this protein in potato, was directly correlated with the level of GNA present in the plants, and that conditions under which the plants were grown affect the levels of GNA expression and subsequent levels of insect resistance.