The commercial preparation of purified diphtheria toxoid primarily depends on the effective separation of the toxin from a large culture volume of Corynebacterium diphtheriae vaccine strain. Tangential flow or cross-flow filtration is gaining momentum in the processing of biologicals. This study deals with the separation of toxins using a microporous membrane (0.45-mum pore size) with operational variables like transmembrane pressure (TMP), cross flow rate and flux. Under the best conditions, >98% recovery was achieved. The data and conclusions presented here have been drawn from process development work employing available equipment.