In order to investigate possible roles of enzymes involved in nucleic acid degradation in tumorigenesis of osteosarcoma, the activities of acid deoxyribonuclease (DNase) and neutral ribonuclease (RNase) in osteosarcoma tissue were determined. Neutral RNases were isolated and analyzed by ion exchange column chromatography to see whether the enzymes unique to the tumor were present. The activity of neutral RNase in serum of patient with osteosarcoma was also determined to find out whether the serum enzyme level could be used as diagnostic or therapeutic marker. 1. The activities of nucleases in osteosarcoma tissue were significantly increased and the positive rates were observed to be over 80% in acid DNase and 9096 in neutral RNase. 2. The activity of serum neutral RNase in patient with osteosarcoma was increased by 77% (p<0.05) and the positive rates was considerably high. 3. Neutral RNases in osteosarcoma tissue were separated by DEAE-cellulose column chromatography into five peaks, two of them were revealed to be unique to the tumor, and one of the peak RNases (peak V) was further separated by phosphocellulose column chromatography into three peaks, two of them appeared to be specific to osteosarcoma. The results indicate that the nucleases could be used as biochemical and diagnostic markers for osteosarcoma.