The hydrochloride salt of a methylhydrazine derivative with antineoplastic and mutagenic activities. Although the exact mode of cytotoxicity has not been elucidated, procarbazine, after metabolic activation, appears to inhibit the trans-methylation of methionine into transfer RNA (t-RNA), thereby preventing protein synthesis and consequently DNA and RNA synthesis. This agent may also undergo auto-oxidation, resulting in the formation of cytotoxic free radicals which damage DNA through an alkylation reaction.