A vaccine containing a heat-killed recombinant Saccharomyces cerevisiae yeast transfected with mutated forms of Ras, an oncogene frequently found in solid tumors, with potential immunostimulant and antitumor activity. Upon administration, GI-4000 vaccine elicits an immune response by stimulating a specific cytotoxic T-cell response against the mutated forms of Ras. This may lead to a destruction of cancer cells expressing a Ras mutation.