The hydrochloride salt of mechlorethamine, a nitrogen mustard and an analogue of sulfur mustard, with antineoplastic and immunosuppressive activities. Mechlorethamine is metabolized to an unstable, highly reactive ethyleniminium intermediate that alkylates DNA, particularly the 7 nitrogen of guanine residues, resulting in DNA base pair mismatching, DNA interstrand crosslinking, the inhibition of DNA repair and synthesis, cell-cycle arrest, and apoptosis. This agent also exhibits lympholytic properties.