The influence of insulin on collagen and total protein production was studied in cultured human skin fibroblasts from patients with Type 1 (insulin-dependent) diabetes and Type 2 (non-insulin-dependent) diabetes, and from non-diabetic subjects. Fibroblast cultures were exposed to various concentrations of insulin (0-10(5) mU/l) for 48 hours in serum-free medium containing tritium labelled proline. With medium glucose at 5.6 mmol/l, cells from diabetic subjects displayed an increase in collagen and protein radio-activities in the extracellular medium in the presence of insulin at 10(2) or 10(3) mU/l. Concentrations above 10(3) mU/l did not stimulate further. In cells from non-diabetic subjects, stimulated incorporation was evident only at insulin 10(3) mU/l. In the presence of high glucose concentration (38.9 mmol/l) in the medium, the lowest insulin level (10(2) mU/l) did not influence collagen production in any cell type, and the effects of insulin on total protein radioactivity was reduced in all cells. The enhancement by insulin of collagen secretion, and the higher sensitivity of cells from diabetics to this insulin action, is of interest in relation to the possible role of insulin in the accelerated atherosclerosis observed in diabetics.