The capacity of LDH-elevating virus to elevate serum colony-stimulating activity may not be unique, for even in the limited series of tests of other viruses in the present study, Newcastle disease virus was found also to stimulate elevation of colony-stimulating activity. Many viruses and perhaps other infectious agents may share a capacity to induce the development of elevated levels of colony-stimulating activity in the serum of the host animals. This would be in agreement with survey **data** on colony-stimulating activity in mouse and human sera, where serum activity has been found to be very low in germfree mice (11), to be high in mice from colonies with a high endemic rate of disease caused by pneumonia virus of mice, and to be elevated in humans in the acute stages of certain viral and protozoal infections. 1 The biological function of the colony-stimulating factor in the intact animal remains uncertain but its properties and occurrence suggest that it may be a normal humoral factor regulating leukopoiesis and that serum levels of the factor are elevated in acute infections when white cell changes are occurring as part of the host response to infection.