Oxyhemoglobin in dilute aqueous solution is rapidly converted by ultrasonic irradiation into methemoglobin, which in turn is gradually destroyed, as shown by the disappearance of its optical absorption peak at 4050 A. Hematoporphyrin similarly suffers partial destruction by ultrasound. On the basis of experiments in various chemical environments, the previous results are ascribed largely to the nitrous and nitric acids produced by ultrasonic cavitation in water containing dissolved air. Adding 0.1 ml ether/25 ml of solution causes the sonochemical change to be from oxyhemoglobin into carboxyhemoglobin, instead of into methemoglobin. Ultracentrifuge studies indicate that ultrasonic treatment of hemoglobin in more concentrated solution also causes some splitting off of the heme from the globin. A comparison of the reported effects of x rays on hemoglobin with those found for ultrasound shows that there are differences as well as similarities. The ultrasonic frequency used was 400 kc per sec, and the acoustic power delivered into the reaction vessel in a focused beam was about 20 w.