More extensive complementation tests than those performed initially (Webber and de Serres, 1965) on a series of 832 X-ray-induced specific-locus mutations in the adenine-3 (ad-3) region of a two-component heterokaryon (H-12) of Neurospora crassa (de Serres, 1989a) showed that unexpectedly high frequencies of specific-locus mutations in the ad-3 region have additional, but separate, sites of recessive lethal (RLCL) damage in the immediately adjacent genetic regions. The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than that expected on the basis of target theory and classical models of chromosome structure during interphase (de Serres, 1989a). Genetic fine structure analyses, by means of homology tests with tester strains carrying genetic markers in the ad-3 and immediately adjacent regions, have been performed to map the presumed multiple-locus mutations. In a previous paper (de Serres, 1989c), X-ray-induced irreparable ad-3 mutants of the following genotypes and numbers (ad-3A or ad-3B were analyzed, and the high frequency of multiple-locus mutations was confirmed. In the present paper, X-ray-induced irreparable ad-3 mutants of the following genotypes and numbers (ad-3A ad-3B, ad-3A ad-3B nic-2, and ad-3B nic-2 have also been subjected to the same genetic fine structure analysis. These experiments, in the previous (de Serres, 1989c) and present papers, were designed to determine the extent of the functional inactivation in the ad-3 and immediately adjacent genetic regions in individual mutants classified as presumptive multilocus deletions or multiple-locus mutations.