Therapy with dialysable leukocyte extract repeated 6 times, had a beneficial effect on the impaired functions of mononuclear leukocytes in Hodgkin's disease. The decreased phagocytosis and chemotaxis of monocytes increased almost to the values of healthy controls. There was no significant change in the C3b receptor activity of the patients' monocytes but their pathologically increased EA rosette formation showed some correction during dialysable leukocyte extract therapy. The number of T cells bearing histamine and IgG Fc receptors was reduced initially and increased during therapy but this effect was only temporary. The results suggest that the stimulating effect of dialysable leukocyte extract on cellular immunity was due partly to the correction of mononuclear phagocyte functions rather than an effect on lymphocyte subpopulations in Hodgkin's disease. The beneficial effect of transfer factor on mononuclear phagocyte function may affect the rate of tumour progression and reduce the number of severe infections in patients with Hodgkin's disease.