The kinetics of myeloma cells at diagnosis and during complete remission were compared on 4 IgG K patients. We were able to study experimentally the kinetics of the minimal tumor mass because we prepared anti-idiotypic sera specifically directed against the hypervariable region of the M protein. We have shown that the residual myeloma cell labelling index during remission is usually lower than that observable on diagnosis. This finding conflicts with the current opinion on minimal tumor mass kinetics. Therefore, it is suggested that a new therapeutic strategy should be adopted in the complete remission phase.