Treatment of diabetes by s.c.injections of insulin co-crystallized together with protamine (NPH-insulin) or as insulin zinc crystals together with amorphous insulin precipitates (lente insulin) have been used for decades in order to provide the insulin molecule with prolonged action.In order to engineer insulin analogues with a slower and more predictable profile when administered by s.c.injection,we have examined the effect of insulin acylation.LysB29 was found to be the most advantageous position for acylation by either fatty acids or by various derivatives of cholic acid.It appears that the cholic acid derivatives can affect the physiochemical properties of insulin leading to new molecular forms of the molecule in the presence of zinc ions and phenol.