Loss of genetic information from a number of specific regions of the genome has been documented in primary human gliomas. Recently loss of heterozygosity or nullizygosity of the IFN beta 1 gene has been found in glioblastomas. We used Restriction Fragment Length Polymorphism (RFLP) analysis in order to screen the frequency of the loss of this genes in glial tumors of malignancy grades I-IV. Nullizygosity for IFN beta 1 was detected in 8/30 (27%) of glioblastomas (malignancy grade IV) and loss of heterozygosity in a further two cases (7%). In total, 33% of these tumors lost least one copy of the IFN beta 1 gene. Among the 10 anaplastic gliomas (grade III), 2 (20%) showed loss of one copy of the gene which none of the 7 low grade gliomas (grades I or II) showed any evidence of loss of IFN beta 1 alleles. The loss of the IFN beta 1 gene would appear to be a late event associated with the development of an increasingly malignant phenotype in human gliomas and to be confined to gliomas of malignancy grade III or IV.