Retinoids varied in their capacity to induce differentiation in HL-60 cells in this order: Ro 13-6307, tretinoin, isotretinoin, acitretin and Ro 13-7410 (high to low). In contrast, retinoids lacking a polar carboxylic acid, such as temarotene and Ro 14-6113, were inactive. Various cytokines had no differentiation-inducing effect by themselves. However, the addition of cytokines to retinoids increased differentiation. Combined with tretinoin, cytokines increased differentiation in this order: interferon (IFN) gamma, granulocyte colony-stimulating factor, interleukin-1 alpha (IL-1 alpha), IL-4, tumour necrosis factor alpha and IFN-alpha. Combination of cytokines with isotretinoin, acitretin, Ro 13-7410, and Ro 13-6307 showed a similar pattern of potentiation to that of tretinoin. Temarotene or Ro 14-6113 did not induce differentiation, alone or with cytokines. Combinations of cytokines were not synergistic in the presence of retinoids; antagonism was even observed. In U937 cells, lower levels of differentiation-induction were observed. Potentiation of the differentiation-inducing effect of retinoids by cytokines might indicate a clinical differentiation therapy of tumours.