The close linkage between the PstI-restriction fragment length polymorphism (RFLP) disclosed by the L2.7 genomic DNA probe and the Kidd blood group locus is described. The maximum lod score is +8.53 at recombination fraction theta = 0.03. The upper probability limit of the recombination fraction is theta 1 = 0.11. The L2.7 probe, previously assigned provisionally to chromosome 17, is by the present study assigned to chromosome 18. This also assigns the Kidd blood group locus (JK) to chromosome 18. Accepting previous deletion mapping, the shortest regions of overlap (SRO) for JK is 18q11-12, whereas one of our hybrids assigns L2.7 to 18p11-pter, suggesting centromeric localisation of the linkage group. JK has been assigned previously to chromosome 2 because of its provisional linkage to IGK which in turn has been mapped to 2p12. Our own JK-IGK linkage data do in fact support the previous positive lod scores at high recombination fractions (total lods +4.12 at theta1 = 0.30). No obvious explanation for the conflicting gene mapping data is found.