There is disclosed a method of treating a cancerous condition in a person in need of such treatment, which comprises enhancing the effectiveness of the person's immune system by the administration to such person of at least one aliphatic sulfur compound, preferably a sulfur-containing amino-acid derivative having the formula (I) ##STR1## in which A is hydrogen or a carboxymethylene --CH.sub.2 CO.sub.2 H group, B is hydrogen or an alkyl group having 1 to 3 carbon atoms, and X is hydrogen, or an acyl group --CO--R in which R is an alkyl group having 1 to 3 carbon atoms, or a pharmaceutically acceptable salt of such compound.