The recent availability of synthetic human promsulin through recombinant DNA technology has permitted detailed studies of its actions on metabolism to be undertaken, as well as giving researchers the opportunity to develop methods for measurement of proinsulin in various metabolic conditions. Imtally, this project focussed on the use of rat hepatocytes as a model system for investigating the effect of proinsulin on 3-hydroxy-3-methylglutaryl coenzyme. A reductase, the rate limiting enzyme controlling endogenous cholesterol synthesis An ‘in vitro’ bioassay technique, based on the effect of proinsulin on enzyme activity, was developed to compare the level of proinsulin in serum from nondiabetic subjects with that from a group of diabetic patients. Higher levels of serum proinsulin were observed in the latter group Subsequent work investigated the contribution of biologically active proinsulin to total immunoreactive proinsulin in these patients. A two-site enzyme linked immunosorbent assay was developed using commercial anti-insulin and anti C-peptide immunoglobulins to determine serum proinsulin. The level of immunoreactive proinsulin in diabetic patients was shown to be approximately three-fold higher than the level of bioactive proinsulin as obtained by the bioassay technique. The advantages and disadvantages of both these methods will be discussed.