In this work, the effect of supplementing liquid cellulase production media (CPM) with methylxanthines (aminophylline, caffeine and theophylline), with and without the addition of glucose, on the secretion of cellulases by Penicillium echinulatum strain 2HH (wild-type) and the derived mutant strain 9A02S1 was studied. When compared with unsupplemented CPM, both strains produced higher beta-glucosidase and filter paper activities (FPAs) in CPM supplemented with 1 mu mol l(-1) of caffeine but lower activities with 5 mu mol l(-1) of caffeine. With theophylline only, strain 9A02S1 produced higher beta-glucosidase and FPAs, while aminophylline produced no effect on the cellulase activity of either strain. Supplementation of CPM with 0.5% (w/v) of glucose plus caffeine resulted in higher beta-glucosidase and FPAs being produced by strain 2HH, but not strain 9A02S1, than in CPM supplemented with 0.5% (w/v) of glucose only. These results indicate that different concentrations of caffeine and theophylline can increase the beta-glucosidase and FPAs produced by P. echinulatum strains 2HH and 9A02S1. The results suggest that some methylxanthines, in adequate concentration, can be used as media components to increase cellulase production.