The acid mucins contained in goblet cells of the descending colon of 34 male Sprague-Dawley rats were histochemically labeled by Alcian blue pH 2.5 and quantified in an image analyzer (Cortex Controller). Twenty-two of these 34 rats were treated with 1,2-dimethylhydrazine (DMH) suspended in EDTA solution as a stabilizing agent and the remaining 12 rats with EDTA only. Of the 22 DMH-treated rats, 11 had concomitantly an adenocarcinoma elsewhere in the colon and the remaining 11 rats had no colonic tumors despite DMH treatment. The results indicated that Alcian blue-positive areas occupied 34.5% of the mucosa of the descending colon in tumor-bearing rats, and 35.2% in non-tumor-bearing DMH-treated rats. For EDTA-treated rats the percentage of mucosa occupied by Alcian blue-positive cells was 48.1%. The difference between DMH-treated rats (with or without tumors) and EDTA-treated rats was significant (p less than 0.001). These results suggest that the decrease of Alcian blue areas is related to the protracted treatment with DMH. Whether the decrease in acid mucins is induced by the carcinogen per se or whether it represents a true biochemical premalignant change at the cytoplasmic level remains to be elucidated.