The autosomal dominantly inherited disease neurofibromatosis type 1 (NF1) is caused by mutations of a large gene comprising 59 exons, which code for a protein with 2818 amino acids called neurofibromin. Employing an expressed polymorphic site in exon 5 of the neurofibromin gene, the expression of its alleles was analysed quantitatively by scanning radioactive RT-PCR fragments of this exon prepared from the RNA of fibroblast cell cultures from 15 NF1 patients and of white blood cells from one NF1 patient. Thirteen of the RNA preparations yielded unequal amounts of the allelic messages. The deviations of the expression ratios (A2:A1) from 1.0 ranged from -0.9 to +25.8. The allelic messages were equally represented in the RNA preparations from five informative healthy donors. Apart from fibroblasts this phenomenon could also be detected in keratinocytes, melanocytes from normally pigmented skin and melanocytes from a café-au-lait spot of one patient. Only one of three patients affected by stop mutations exhibited unequal allelic expression. When nuclear RNA from 10 of the 13 patients was examined, equal amounts of the primary transcripts were found (average ratio A2/A1: 1.08 +/- 0.07 S.E.M.), indicating that unequal expression on the level of mRNA was not caused by mutations affecting transcriptional regulation. The ratio of the amount of neurofibromin to that of p120 GAP did not seem to be correlated with the extent of unequal allelic expression.