Viable Azotobacter vinelandii cells were immobilized in composite agar gel layer/microporous membrane structures and tested for alginate production from sucrose during batch incubation. Microporous membranes with varying pore sizes (0,22-3.0 mum) were used in the composite structures. Whatever the membrane pore size (MPS), the duration of the production period was short (ca. 100 h), most sucrose remaining unconsumed by immobilized organisms. The amount of alginate recovered (0.5-0.9 g dm(-3)) and volumetric productivity increased slightly with the MPS, but the average production yield was fairly stable with a mean value of 0.24 g alginate produced g(-1) sucrose consumed, i.e. noticeably higher than that of free-cell counterparts (0.09 g g(-1)). Furthermore, immobilization was shown to favour the production of higher molecular weight (MW) polysaccharide macromolecules and to stabilize the MW distribution of alginate produced as compared to conventional free-cell cultures grown in shake flasks. (C) 2002 Elsevier Science B.V. All rights reserved.