In cell lines derived from patients with multiple myeloma (MM) we have found an elevation in the amount of the c-myc protein which is not accompanied by an increase in the level of mRNA or a change in the half-life of the protein. There is a 3.4 fold enhancement in the degree of association of the c-myc message with polysomes. This is not accompanied by an alteration in polysome size or a change in the transit time of the c-myc mRNA on the polysomes thus suggesting that there is in increase in the degree of mobilisation of the c-myc message. Sequencing of the c-myc 5'UTR has revealed the presence of a mutation in all the MM cell lines studied and we demonstrate that this mutation causes altered binding of cellular proteins to this RNA species.