A bilayer strip, cut from a thin layer of cross-linked polyacrylamide gel cast on to cellulose tissue, forms an open circular loop whose ends are close together. Shrinkage of the gel, in response to the osmotic pressure of a non-penetrating solution, causes a proportional separation of the ends of the loop. This is measured with a microscope and micrometer eyepiece. 2. The resulting effective sensitivity is about 30 times that of the Sephadex-bead osmometer (Ogston & Wells, 1970), i.e. of the order of 5Pa, comparable with that of a membrane osmometer. Use of gel up to 70% (w/v) allows the measurement of molecular weights, as low as 1500 in favourable cases, with an accuracy of 1-2%. The useful range of osmotic pressure is up to 5kPa. A single measurement requires 0.5ml of solution. Equilibration is completed in 20-30min. 3. The method is illustrated by measurements on human serum albumin, ovalbumin, cytochrome c, samples of dextrans, polyvinyl alcohol, and polyethylene glycols 6000 and 1000.
