Microfiltration membranes of poly(L-lactic acid) (PLLA) were prepared from PLLA-1,4-dioxane-water solutions via the thermally induced phase separation process. Ternary phase diagrams for this system, determined at 10, 48, and 80 C, indicate that essentially polymer-free droplets form in a matrix phase of PLLA-1,4-dioxane-water solution. Rapid solidification of the PLLA after the initial liquid-liquid phase separation was necessary to obtain membranes with open pores at the membrane surface and low permeation resistance. Using filtration of cell suspensions, the effective pore size of the best membrane formed in this study was found to be between 0.6 and 4.4 mum. (C) 2004 Elsevier B.V. All rights reserved.