Actinorhodin production by Streptomyces coelicolor A3(2) immobilised naturally in a porous support material was investigated in a 20 1 bioreactor. The effect of pH-control on actinorhodin fermentation was conducted by performing different strategies. The combination of pH-control and immobilisation of the cells produced a different result than pH-control with freely suspended cells. The control of pH at 7.2 in an immobilised cell system altered the physiology of the cells shifting the product, actinorhodin, to another pigmented secondary metabolite, undecylprodigiosin. Although the cells were biologically active in such an environment, they were unable to produce actinorhodin. In the freely suspended cell system, however, actinorhodin biosynthesis was not hindered by controlling the medium pH. (C) 2002 Elsevier Science Ltd. All rights reserved.