Laetiporus sulphureus is a source of alpha-1,3-glucan that can substitute for the commercially-unavailable streptococcal mutan used to induce microbial mutanases. The water-insoluble fraction of its fruiting bodies from 0.15 to 0.2% (w/v) induced mutanase activity in Paenibacillus sp. MP-1 at 0.35 mu ml(-1). The mutanase extensively hydrolyzed streptococcal mutan, giving 23% of saccharification, and 83% of solubilization of glucan after 6 h. It also degraded alpha-1,3-polymers of biofilms, formed in vitro by Streptococcus mutans, even after only 3 min of contact.