Many epithelial renewal tissues in vertebrates are organised into structural proliferative units. We have examined the effect of IGF2 dose on the structure of structural-proliferative units in skin and colon. The mouse strains used were the Igf2 knockout, wild type and K:Igf2, a transgenic in which Igf2 is overexpressed under control of a keratin promoter. For both skin and colon, the histological organisation of structural-proliferative units was unaltered with increasing IGF2 dose, although there was a higher fraction of dividing cells in the proliferative compartment. In the colon an increase in IGF2 dose increases the overall area of the epithelium. This is due to an increase in the number of crypts with no change of cell size or of crypt area. Growth stimulation appears to be due to a reduction in the duration of crypt fission. The conclusion is that the IGF2 pathway can stimulate the multiplication of colonic crypts independently of stimulating increased cell proliferation. The results for the skin are consistent with this. An increase of IGF2 dose increases the proportion of dividing cells in the basal layer, the thickness of the epidermis and the total area of the epidermis. By comparison with Drosophila, these results show no effects on cell size, but do show the possibility of inducing disproportionate growth. These differences may represent properties of the SPU organisation that is characteristic of vertebrate tissues.