WE read your interesting editorial1 dealing with our work on the major transplantation antigens of rhesus monkeys, the RhL-A system. Your communication ends: 'The previously held concept of H-2 genetic structure is now very much open to doubt. If further work shows that the RhL-A system also consists of two segregant series of antigens it is bound to undermine even more the concept of H-2 as a system of multiple, pseudoallelic antigens'.