Inositol-phosphates, glucosamine and glucose-6-phosphate blocked the effects of insulin on target protein phosphorylation in adipocytes, but the unsubstituted or sulphated derivatives of inositol or of glucose, or N-acetyl-glucosamine were without effect. The insulin stimulated tyrosine phosphorylation of the insulin receptor was not affected. The sugar-phosphates inositol-phosphate and glucose-6-phosphate did not enter into the cells. They also blocked the insulin-like effects of a potential second messenger of insulin, a phospho-oligosaccharide (POS), which has previously been shown to mimic the effects of insulin on protein phosphorylation in intact cells.