We have applied restriction fragment length polymorphism analysis to a 30-member panel of primary glioma DNAs, which had been previously examined for loss of genetic information (C. D. James, E. Carlbom, J. P. Dumanski, M. Hansen, M. Nordenskjold, V. P. Collins, and W. K. Cavenee, Cancer Res., 48:5546-5551, 1988), to determine the frequency and sublocalization of loss of genetic information from chromosome 9. We have also utilized scanning densitometry for dosage determination of the 9p-localized interferon alpha and interferon beta-1 genes among these same tumors. Our results reveal the following: (a) for those cases in which loss has occurred, the region of common loss lies on the short (p) arm of the chromosome; (b) loss of genetic information from the short arm of chromosome 9 occurs frequently in glial tumors of intermediate (anaplastic, grade III) and high (glioblastoma, grade IV) histological malignancy (10 of 20 cases) but not in tumors of low (grade II) histological malignancy (0 of 10 cases); (c) tumors with 9p deletions are hemi- or nullizygous for interferon beta-1 and the interferon alpha gene cluster; (d) cases of interferon nullizygosity occur exclusively among tumors of highest histological malignancy (glioblastoma). These data, especially the determination of a region of nullizygosity, suggest proximity to or residence within a gene(s) whose function(s) is (are) critical to the suppression of the malignant evolution of glial tumors.