The neurofibromatosis type 1 (NFl) protein contains a region of significant sequence similarity to ras p21 GTPase-activating protein (GAP) and the yeast IRA1 gene product. A fragment of NFl cDNA encoding the GAP-related domain (NFl GRD) was expressed, immunoaffinity purified, and assayed for effects on N-ras p21 GTPase activity. The GTPase of wild-type fas p21 was stimulated by NFl GRD, but oncogenic mutants of ras p21 (Asp-12 and Val-12) were unaffected, and the GTPase of an effector mutant (Ala-38) was only weakly stimulated. NFl GRD also down-regulated RAS function in S. cerevisiae. The affinity of NFl GRD for r8s p21 was estimated to be 250 nM: this is more than 20-fold higher than the affinity of GAP for ras~ 21. However, its specific activity was about 30 times lower. These kinetic measurements suggest that NFl may be a significant regulator of ras p21 activity, particularly at low ras p21 concentrations.