Abstract: The invention is related to a process for the treatment of humans suffering from undesired urotoxic side effects caused by cytostatically active alkylating agents administered to them against malign tumors, comprising administering a pharmacologically acceptable salt of a mercapto alkane sulfonic acid having the general formulaHS--alk--SO.sub.3 4to the human treated with the cytostatically active alkylating agent, in an amount ranging from 20% of the weight of the alkylating agent to the highest tolerated dosage of the salt of the mercapto alkane sulfonic acid, simultaneously or separately in combination with the alkylating agent.