We have investigated which of the cytokines that are relevant in the in vitro growth of multiple myeloma (MM) malignant plasma cells are actually produced in vivo by MM patients. To this end, we have measured the levels of IL-1 beta, IL-3, IL-4, IL-6, IL-7, IL-8 and tumour necrosis factor-alpha (TNF-alpha) both in sera and in the supernatant of bone marrow (BM) stromal cell cultures from patients with MM and monoclonal gammopathy of undetermined significance (MGUS). The significance of our findings is three-fold. First, IL-6 and IL-8 are produced by MM BM stromal cells, while IL-1 beta, TNF-alpha, IL-4 and IL-7 are not. Second, IL-3 is the only cytokine consistently raised in serum samples: we have also detected low levels of serum IL-6 in a minority of cases, usually in advanced stage of the disease. Third, MM BM stromal cells are active IL-6 and IL-8 producers, while both normal and MGUS BM stromal cells are low producers, thus suggesting that in the BM of MM a number of environmental cells, that would normally be quiescent, are instead activated and that, in MM, activated BM stromal cells play an active role in supporting the progressive expansion of the B cell clone.