A family is described with five affected males segregating a new gene for non-specific X linked mental retardation (MRX). Linkage analysis localised the gene at Xq28-qter. The maximum lod score was 2.89 with DXS52 (St14) at theta = 0.0. A recombinant was observed with DXS304 (U6.2) defining the proximal limit to the localisation. No evidence for linkage was determined using markers at several points along the remainder of the X chromosome, including the regions known to contain MRX1 and MRX2. This delineates the third gene for non-specific X linked mental retardation, MRX3.