Families can be genotyped with respect to their inheritance of the units of inheritance of the HL-A system (haplotypes). The conclusion that skin exchanged between siblings who are genotypically identical with respect to HL-A is accepted for prolonged periods has been confirmed. The antigenicity of a haplotype can be determined by skin transplantation between siblings. Some haplotypes are rapidly reacted against, others elicit little effect alone but may be effective in combination. A possible recombinant was recognized serologically. The hypothesis was strengthened by the rejection times of skin grafts. Guidelines are suggested for the confirmation of recombination by (a) skin grafting and (b) the production of antibodies against the recombinant haplotypes, and for starting a linkage map.