An immunoconjugate consisting of a human IgG1 monoclonal antibody directed against CD27L conjugated, via a non-cleavable linker, to the cytotoxic agent maytansinoid DM1, with potential antineoplastic activity. The monoclonal antibody moiety of this immunoconjugate binds to CD27L on tumor cell surfaces. After internalization, the DM1 moiety binds to tubulin, thereby disrupting microtubule assembly/disassembly dynamics and inhibiting both cell division and proliferation of cancer cells that express CD27L. CD27L, a type II transmembrane protein and member of the tumor necrosis factor family, is a co-stimulatory molecule constitutively expressed on a subset of activated T-cells, B-cells, and dendritic cells, which is overexpressed in certain tumor cell types.