OBJECTIVES: I. Determine the maximum tolerated dose of mitomycin when administered with irinotecan in patients with advanced solid tumors. II. Determine whether the pharmacokinetic profile of irinotecan is altered by prior administration of mitomycin in this patient population. III. Determine the effect of irinotecan and mitomycin on the expression of DT-Diaphorase and the Topo I gene. IV. Determine the preliminary antitumor activity of this regimen in these patients.