Abstract: An apparatus and method for the inactivation of infectious organisms such as viruses, bacteria, fungi and protozoa, and especially for the inactivation of human immunodeficiency virus in proteinaceous material such as blood and blood products, without adversely affecting the normal physiological activity of the material, by contacting it for a time interval of only about 16 seconds with an ozone-oxygen mixture having an ozone concentration of only about 27 .mu./ml. The apparatus includes a gas-liquid contact apparatus through which the material and ozone-oxygen mixture flow in contacting, counter-current relationship, and an ozone generator which produces an ozone-oxygen mixture having a resonant frequency of about 7.83 Hz. The apparatus and method of the invention provide precise control of the concentration of ozone and the contact time between the material to be treated and the ozone-oxygen mixture.