Complex coacervation of albumin and alginic acid has been investigated to characterize this process, and to prepare a microencapsulation system suitable for the encapsulation of live cells, protein and polypeptide drugs. The optimum conditions of pH, ionic strength and total polyion concentration were in accordance with predictions based on the method of Burgess & Carless (1984). Albumin/alginic acid complex coacervation appears to fit the Vies-Aranyi model for complex coacervation. Coacervation was limited compared with other polypeptide/polysaccharide systems such as gelatin and acacia, with albumin/alginic acid complex precipitates rather than complex coacervates forming under certain conditions. In particular coacervation was limited to concentrations below 0.5% w/v. At concentrations between 0.35 and 0.5% w/v both complex coacervation and precipitation occurred, and at concentrations above 0.5% w/v only precipitation was detected. The albumin/alginic acid complex coacervate is very viscous and this together with the limited conditions governing the occurrence of coacervation makes this system unsuitable for the preparation of microcapsules.