The cellulose fraction in tomato pomace was hydrolysed using Trichoderma reesei, and the resultant sugars were fermented with Propionibacterium shermanii to produce vitamin B-12. A multifactorial experiment revealed that aeration of the culture of T. reesei gave substantial improvements in cellulase activity as did higher concentrations of available nitrogen, but a rapid drop in pH appeared to inhibit extensive hydrolysis; after 14 days, the maximum level of cellulose degradation was only 34.4% of the total available, and the highest level of reducing sugars achieved was 15 g l(-1). When flasks with the latter concentration of reducing sugars were inoculated with P. shermanii, 11.1 mg l(-1) of B-12 were produced under optimum conditions. If the degree of hydrolysis of the cellulose could be increased, then sufficient vitamin B-12 might be generated to justify extraction but, even if purification does not prove to be economically feasible, a fermented tomato pomace (dried) with 50-55 mg kg(-1) or more of B-12 could prove a useful feedstuff for animals.