The relationship of wool growth to cell proliferation in the follicle bulb and to the subsequent migration and growth of the fibre cortical cells was investigated in 10 Peppin Merino sheep. These sheep had been maintained on a low, medium or high level of nutrient intake to ensure a wide range in wool growth. The number and mitotic activity of the germinal cells in the follicle bulb were determined after administration of colchicine. Cortical cell size was measured following isolation of the fibre cells by acid-treatment of wool. The average fibre production of the follicle varied from 4.1 x 10(4) to 13.2 x 10(4) micron3/day in these sheep. There were also substantial differences between sheep in the mitotic activity of the germinal cells in the bulb, the rate of cell proliferation being highly correlated with the average daily fibre production of the follicle (r = + 0.88, n = 10). However, the size of the germinal cell population differed from sheep to sheep and was not closely related to the level of fibre production (r = + 0.48, n = 10). The average turnover time of these cells was inversely related to fibre production and varied from 41.6 to 19.4 h (r = -0.82, n = 10). Multiple regression analysis of the data showed that the average daily fibre production of the follicle was largely determined by the number of germinal cells present in the bulb and their rate of proliferation (R = +0.95, n = 10). Variations in cell turnover time and in cortical cell size were not significant in influencing the rate of fibre production. In these sheep, the average cortical cell varied in size from 658 to 1279 micron 3 and the positive correlation (r = + 0.83, n = 10) found between cell size and fibre production is considered to merely reflect an allometric relationship. The proportion of germinal cells contributing to the fibre cortex was found to be small and variable, ranging from 9.4 to 17.8%. Furthermore, this proportion was not related to the nutritional level of these sheep, and it is thought that the variability in the distribution of cells to the fibre may be attributed to genetic differences between sheep.