The retinoids: all-transretinoic acid (tretinoin), 13-cis retinoic acid (isotretinoin) and the aromatic retinoids etretinate and acitretin have a preventive and therapeutic effect on chemically-induced tumours. Clinically, retinoids have shown variable effectiveness in therapy and/or prevention of oncological diseases of skin, head and neck, lung, bladder, vulva and bone marrow. With a few exceptions, monotherapy with retinoids has not been satisfactory. Similarly, monotherapy with interferon alpha has been used successfully only for some specific indications. Retinoids have a marked differentiation-inducing effect which may contribute to their therapeutic effect. Experiments were carried out in transformed cell lines to test the combination of retinoids with interferon alpha and other cytokines on differentiation. In HL-60 cells, an acute promyelocytic leukaemia cell line, induction of differentiation was determined by induction of an oxidative burst potential. Retinoids showed the following order of activity: tretinoin greater than isotretinoin greater than acitretin. Cytokines had no differentiation-inducing effect by themselves. However, the addition of the following cytokines to retinoids potentiated the retinoid-induced differentiation: IFN alpha, IFN beta, IFN gamma, TNF alpha, G-CSF, IL-1 alpha and IL-4. In experiments with HL-60 or other cell lines, the pattern of differentiation-induction was always dependent on the particular retinoid/cytokine combination. IFN alpha provoked a marked potentiation of retinoid-induced differentiation. The combination of the antiproliferative and differentiation-inducing effect of the retinoids together with the antiproliferative, immunostimulatory and differentiation-potentiating effects of IFN alpha suggests that this combination might be a particularly promising treatment for neoplastic diseases.