Pulsed proton nuclear magnetic resonance was used to differentiate between normal and malignant tissues. When the tissue water content varied from 80 to 93%, the tumors exhibited spin-lattice relaxation times (T1) from 0.9 to 1.8 sec. We report also the results obtained on 9-day-old embryos and on liver, brain, and heart from 2-day-old rats. A good correlation between the spin-lattice (T1) and spin-spin (T2) relaxation times and the tissue water content was found for all tissues studied. The relaxation times T1 and T2 and water content in Walker 256 carcinoma and its lymph node metastasis were quite similar.