Several new 5-substituted 1-phenylpyrazole [3,4-d] pyrimidine derivatives were synthesized and tested for antitumor activity. Compound 17, which has 3,4-dichlorophenyl moiety at N5 of the pyrazole pyrimidine ring, shows a strong activity against L-1210 leukemia (152%). On the other hand, compounds 15, 17 and 21 were the most active against sarcoma Sa-180. It was observed that many derivatives of pyrazole [3,4-d] pyrimidine possess different biological, especially antitumor properties [1-6, 9, 10].