Microfiltration membranes of poly(L-lactic acid) (PLLA), which is a major biodegradable plastic, were prepared from PLLA-1,4-dioxane-water solutions via the thermally induced phase separation. The resulting membranes are used to retain bacterial cells and permeate protein molecules. Only 20% of the bacterial cells in cell suspensions were retained by the membranes prepared from 10-12.5% polymer solution. Increasing the polymer concentration from 12.5 to 15% markedly elevated the bacterial cell retention to 60%. The retention further increased to 90% by elevating the casting temperature from 40 to 50 degrees C. By drying the polymer solution for 2 min before quenching, the cell retention reached 99%. Casting at temperatures of 60 degrees C or higher and drying for periods of 5 min or longer made the membrane uneven and/or increased the membrane resistance. By drying the polymer solution before quenching, the porous structure near the membrane surface was altered so that the membrane retains the bacterial cells. The membrane retained bacterial cells in the manner of a screen filter. The membranes developed in this research will be useful as a pre-filter in biochemical processes to enhance the life time of the final filters and to reduce industrial wastes due to being made of a compostable polymer.