The recovery of penicillin acylase from E coli by a new reverse micellar treatment is described. The results are compared with the cell disruption by ultrasound followed by reverse micellar extraction. The process gave selective extraction of penicillin acylase directly from the periplasmic space of E coli without disrupting the cells. Unlike ultrasonication which breaks open the cells entirely, making subsequent processing dif®cult and expensive, reverse micellar treatment of cells gave a moderate recovery of 60% of enzyme activity in a highly pure form.