Microfiltration membranes that can be composted after service were developed from biodegradable polyesters, poly(L-lactic acid) (PLLA) and poly(epsilon-caprolactone) (PCL). The membranes were formed via the thermally induced phase separation method. A 10 wt% PLLA solution in a mixed diluent of 1,4-dioxane and water (87:13 by weight) was prepared in a flat mold and quenched from 52 degrees C (4 degrees C above cloud point temperature) to 0 degrees C. After diluent extraction, the membrane separated yeast cells (6 mu m) from their suspensions. A PCL membrane formed by the same method did not reject yeast cells. PCL membranes formed by quenching a 16 wt% PCL solution to 0 degrees C and quenching a 10 wt% PCL solution to -196 degrees C did separate yeast cells from their suspensions. The permeation flux was much higher in the filtration of 1 kg.m(-3) yeast cell suspension with the PLLA and PCL membranes formed by quenching a 10 wt% PLLA or PCL solution to -196 degrees C than in the filtration with the PLLA membrane formed by quenching a 10 wt% PLLA solution to 0 degrees C. The higher flux would be due to the lower resistance of the membranes formed by liquid nitrogen quenching (-196 degrees C) and the mode of depth filtration. Porous biodegradable microfiltration membranes prepared from these polymers have the potential to serve as disposable filters in food and biochemical industries.