In vitro and in vivo studies performed during recent years have provided evidence that cytokines exert a major biological role in multiple myeloma (MM). Cytokines control myeloma cell proliferation, differentiation and apoptosis, and are involved in the tumour-induced bone destruction mediated by activated osteoclasts. Interferon-(Inf-) was introduced for therapeutic purposes in MM more than 15 years ago, and other cytokines have been evaluated more recently. Although controversies still exist, it seems appropriate in 1996 to discuss the indications for cytokine therapy in MM, and to correlate results obtained in vitro with the effects observed in vivo.