We developed a gel filtration assay for the determination of glycogen synthase activity in cultured cells or tissue homogenates. Compared to the commonly used filter paper assay, the gel filtration assay resulted in a more than 5-fold reduction of background levels leading to an--at least--twofold increase in precision. These benefits allow the gel filtration method to detect differences of +/-5% in enzyme activity out of 300 microg total cell protein. In addition to high precision and sensitivity, the method's additional salient advantages include lesser expenditure of time and labour and reduced exposure time of the personnel to radioactivity.