Phytohemagglutinin, an extract of the kidney bean, Phaseolus vulgaris, induces in human leukocyte cultures an inhibitor of the cytopathic effects of Sindbis virus. The physicochemical and biological properties of this virus-inhibitor are similar to those of interferon induced by Newcastle disease virus, except for an instability at pH 2 and 10 and at 56 degrees C.