The biochemical and clinical effects of selenium (Na2SeO3) on 1,2-dimethylhydrazine (DMH)-induced colon carcinogenesis in male Sprague-Dawley rats are presented. A 4-ppm selenium supplement to the drinking water was provided before, during, and after 20 weekly injections of 20 mg DMH per kg body weight. Immediately after the 20th DMH injection, part of the rats were sacrificed. The incidences of colon tumors in groups provided selenium before DMH, before and during DMH, and only during DMH treatment were reduced to 39, 43, and 36%, respectively. The incidence in the DMH only control was 63%. Other rats in all treated and control groups were maintained up to 5 months post-DMH treatment. At 10-week intervals throughout the study, selected blood and tissue components were analyzed. The following hematological changes correlated with DMH treatment. (a) Serum glutamic oxalacetic transaminase increased 2-fold (normal, 66 +/- 14 g/dl). (b) Serum alkaline phosphatase increased 24% (normal, 166 +/- 56 units/liter). (c) Serum protein decreased 14% (normal, 6.77 +/- 0.48 g/dl). (d) White blood count increased 2- to 3-fold (normal, 7.7 +/- 2.7 X 10(3)/cu mm). And (e) hemoglobin decreased 67% (normal, 18.1 +/- 1.3 g/dl). The magnitude of these changes varies with each selenium treatment group and with each 10-week analysis period. Provision of 4 ppm selenium doubled both liver and blood selenium levels compared to unsupplemented controls. The effects of selenium and DMH treatments on glutathione peroxidase and beta-glucuronidase activities and on sialic acid are presented. Possible mechanisms by which selenium protects against DMH-induced neoplasia are discussed.