The genomic DNA-probe L2.30 was used to assign D2S1 to 2p23-pter by in situ hybridization. The RFLP revealed by BglII was then used for linkage studies in the Oslo-NHIK families segregating for the acid phosphatase ACP1 protein polymorphism. Evidence for very close linkage was found by a lod score of +17.17 at recombination fraction theta = 0.01. By this close linkage 92 informative meioses could be inferred from the families and with only a single crossover. The upper probability limit to the recombination fraction is 0.06 according to the HGM 8 criterion. No association between ACP1 alleles and D2S1 BglII alleles was found. The Norwegian gene frequencies for D2S1 were A1 (9.0 kb) = 0.65 and A2 (6.3 kb) = 0.35.