Thirteen kinds of 45-mer or 30-mer synthetic oligonucleotides with sequences randomly selected from the cDNA encoding three kinds of protein of Mycobacterium bovis BCG were tested for their antitumor activity in a murine tumor system. Six out of the 13 single-stranded oligonucleotides which contained one or more hexameric palindromic sequences showed strong antitumor activity while the others without palindromic structure did not. Namely, repeated intralesional injections of 100 micrograms of the 6 oligonucleotides caused regression of the established tumor but the other 7 were ineffective. When tumor cells were mixed with 100 micrograms of an effective oligonucleotide and injected into mice, tumor growth was markedly suppressed. These results suggested that palindromic structure is essential for the antitumor activity of the synthetic oligonucleotides.