The effect of several derivatives of ureidosuccinic acid on the growth of Escherichia coli 387, Staphylococcus aureus strain 209 and its mutant UV-2, Sarcina lutea, Candida tropicalis and Neurospora crossa 9863 as pre-screening systems for antitumor activity was studied. It was found that dihydrazide of D,L-ureidosuccinic acid (DHUA) had a marked antibacterial activity. The inhibitory effect of DHUA on N. crassa could not be removed by aspartic acid, ureidosuccinic acid, dihydroorotic acid, orotic acid, uracil or cytosine. DHUA suppressed the growth of Myeloma P-8 by 38%, that of Sarcoma 180 by 12% and that of Yoshida sarcoma by 19%. No effect was found on the growth of Lymphosarcoma Pliss.