We used a microbioassay to study the distribution of bleomycin in rat brain tumors induced in newborn Sprague-Dawley rats with 1-ethyl-1-nitrosourea (ENU, 50 mg/kg s.c.). Upon suspected successful tumor induction bleomycin (0.1 g/kg i.v.) was administered, and 2 hr later bleomycin concentrations in major organs and tumor tissues were bioassayed using Bacillus subtilis PCI 219 IMC. To determine their histology, the tumors were stained by the immunofluorescence- or immunoperoxidase method using antiserum to astroprotein in addition to the conventional staining methods. There were 11 gliomas each of the brain and spinal cord, 14 schwannomas of the trigeminal nerve and 4 adenomas of the pituitary gland; they developed within 8 (gliomas), 7.3 (schwannomas) and 15 (adenomas) months on average after ENU treatment. The bleomycin concentration and the tumor:plasma concentration ratio were 7.69 +/- 2.84 micrograms/g and 0.13 +/- 0.05 (brain gliomas), 7.10 +/- 3.15 micrograms/g and 0.27 +/- 0.12 (spinal cord gliomas), 5.40 +/- 1.41 micrograms/g and 0.23 +/- 0.05 (schwannomas), 4.83 +/- 1.05 micrograms/g and 0.21 +/- 0.08 (adenomas). Normal brain- and spinal cord tissues scarcely contained bleomycin.