The discoveries of the last two decades have revealed that some important antitumor substances (chalones), hormones, releasing factors, kinins, antibiotics, and behavior-inducing substances, possessed a peptide structure. It has already been proved that some of them are present in urine, which may be a convenient source for their isolation. The general aim of this study is the isolation, purification, identification, and determination of the biological, therapeutic, and diagnostic significance of urinary peptides, especially their effect on normal and neoplastic cell growth. The peptides will be isolated by ethanol extraction, gel filtrations, paper and thin-layer chromatography, and HVE. Since only a small fraction of the urinary peptides is expected to have functional significance, a battery of bioassays will be set up to evaluate the activity on various normal and neoplastic cells (DNA, RNA, and protein synthesis), animal tumors, intestinal and vascular smooth muscle, cardiac function, endocrine effects, metabolic reactions, and behavior. The peptides proved active will be further identified by amino acid analysis and sequence determination and will be reproduced by synthesis. According to preliminary experiments, 10% of the 119 peptides studied up to now inhibit DNA, RNA, or protein synthesis in cultures of human osteosarcoma cells. Four substances of this group showed no inhibitory action on DNA synthesis in normal cells (human embryonal fibroblasts) and no effect on smooth muscle or heart; they have been selected for further study.