This study is mainly concerned with establishing a reliable method of quantitative analysis of volumetric flux and transmission in ultrafiltration membranes. Experiments were conducted in a newly developed vortex flow configuration and the data for transmission and volumetric flux obtained as a function of pressure and rotational speed of the membrane module were analyzed. Membranes of molecular weight cut-off of 10,000 (MX-10) and 100,000 (MX-100) were used with protein solutions of low and medium sized protein molecules in dilute concentrations. A model for transmission which combines a three-parameter model based on irreversible thermodynamics and concentration polarization was used for explaining the experimental results. Based on the values of the parameters, differences in the behaviour of high (MX-100) and low flux (MX-10) membranes were predicted. This model also predicted a minimum in transmission behaviour with flux, which was confirmed by the experimental investigations. The theoretically predicted transmission and the experimental results for both membranes were in good agreement.