Previous studies on X-ray-induced irreparable adenine-3 mutations (designated [ad-3]IR), induced in heterokaryon 12 of Neurospora crassa, demonstrated that they were not recessive and exhibited heterozygous effects in terms of markedly reduced linear growth rates (de Serres, 1965). Complementation tests with a series of tester strains carrying multilocus deletion mutations in the ad-3 and immediately adjacent genetic regions demonstrated that X-ray-induced irreparable mutations map, in the main part, as a series of overlapping multilocus deletion mutations that extend both proximally and distally into the immediately adjacent genetic regions, as well as into the 'X' region (a region of unknown, but essential function) between ad-3A and ad-3B (de Serres, 1968, 1989). Further studies (de Serres and Miller, 1988) have shown that the heterozygous effects of multilocus deletion mutations in the ad-3 region can be modified genetically and biochemically. In the present paper, the heterozygous effects of X-ray-induced multilocus deletion mutations of genotype ad-3A or ad-3B, induced in heterokaryon 12 (Webber and de Serres, 1965; de Serres, 1988, 1989), have been determined. These data show that 57.7% (15/26) of X-ray-induced multilocus deletion mutations covering the ad-3A locus have heterozygous effects, in terms of reduced linear growth rates, in forced dikaryons with a gene/point mutant at the ad-3B locus and 80.0% (20/25) in forced dikaryons with a multilocus deletion mutation covering the ad-3B locus. In addition, 35.1% (20/57) of X-ray-induced multilocus deletion mutations covering the ad-3B locus have heterozygous effects in forced dikaryons with a gene/point mutant at the ad-3A locus, and 100.0% (35/35) in forced dikaryons with a multilocus deletion mutation covering the ad-3A locus. These results demonstrate that the dominant or recessive characteristics of X-ray-induced specific-locus mutations resulting from multilocus deletion mutations are allele specific.