A method is presented for the detection of hybridoma cell growth in vivo by making use of the specific antigen (hapten) recognition properties of the produced monoclonal antibodies. Horseradish peroxidase (HRP)-labeled antigen was used for demonstration of intracellular specific antibodies in hybridoma cells. It appeared that intravenously injected anti-penicilloyl-producing hybridoma cells developed mainly in the red pulp of the spleen; to a lesser extent they were found in the liver.