The transport phenomenon of limiting flux in the steady, parallel plate laminar ultrafiltration of macromolecular solutions with concentration dependent transport properties is investigated. A semiempirical formula for limiting flux is developed that embodies the effect of variable diffusion coefficient in a correction, which modifies the widely used formula developed by Michaels and which is a function of the diffusion coefficient at the gel concentration. An exact numerical calculation is also carried out in which the variation in oth diffusion coefficient and viscosity with concentration is taken into account. The semiempirical formula and numerical calculation for limiting flux for a bovine serum albumin solution of known properties are shown to be in excellent agreement be tween themselves. In addition, the semiempirical formula requires minimum rheological knowledge of the solution, is easy to apply, and is demonstrated to provide a simple method for determining diffusion coefficient at the gel concentration. Predictions using the semiempirical formula are shown to be in good agreement with experimental ultrafiltration data in the literature for bovine serum albumin solutions.