In 54 patients with leukaemia a raised incidence of HL-A9 was noted as well as a markedly increased association between this antigen and HL-A2. This occurred most frequently in patients with chronic myeloid leukaemia. As HL-A2 and HL-A9 are both antigens of the first series it has been suggested that the predisposition to develop leukaemia is controlled by a recessive gene closely linked to the first HL-A locus and in a linkage disequilibrium with HL-A2 and HL-A9. 5 patients also showed definite changes between antigens of the same series, whilst others suffered a partial or total loss in antigenicity. Lymphocytes from 145 controls did not behave in this way, though other patients receiving radiotherapy also ‘lost’ antigens. So it was postulated that such changes resulted from the treatment of the disease rather than the disease itself.