Background: CD137L, a member of the tumour necrosis factor receptor (TNFR) family, is constitutively expressed on activated antigen-presenting cells. In a wide range of solid tumours, over-expression of CD137L has been shown to produce tumour immunity. This was at least partly due to the stimulation of CD8+ CTL. Costimulation by other known interactions was not unconditionally necessary. Anti-tumour effects were even increased when immunotherapy with Interleukin-12 was additionally employed, either systemically or locally. Yet, there is rare data on the effect of CD137L immunotherapy in hematological malignancies. Here, we present data obtained in a murine plasmocytoma model to evaluate the effect of CD137L and IL-12, either as mono- or combination therapy.