A device is descr. which can be used to study the thicknesses of layers of living tissue in situ. Electrical energy in the form of 0.5 msec. pulses at a frequency of 15 mc. is applied to a quartz crystal. A water coupling column transmits the acoustical vibration produced by the crystal into the specimen to be examined. Echoes produced by the acoustical impedance discontinuities of the system are converted into electrical signals by the crystal, amplified and displayed on an oscilloscope with a linear time base sweep. The echograms thus produced may be uni-dimensional (with the transducer fixed) or 2-dimensional (with the transducer moving). Malignant tumors can be distinguished from nonmalignant and from normal tissue by characteristic features of the echo-grams. There were no evidences of harmful effects on living tissues resulting from the appln. of the ultrasonic pulses. The exact mechanisms of the phenomena involved are still obscure, but the method seems promising for the early detection of accessible tumors.