An outbreak of bloody, thickened milk in dairy cows was observed near Princeton, N. J., during the late spring and summer of 1946. From this abnormal milk an agent was transmitted in serial passage to guinea pigs, rabbits, mice, and the chorio-allantois of embryonated eggs. Guinea pigs and rabbits showed a febrile response, while the infection was inapparent in mice and eggs. After transfer in guinea pigs, inoculation of the agent into lactating cows reproduced the essential manifestations of the disease as seen in naturally infected animals. Neutralizing antibodies developed in exptl. and naturally infected animals. Efforts to demonstrate organisms with the microscope or culturally failed, and it was concluded that a virus caused the abnormal milk.