Runting was produced in newborn (C57BL/6 X DBA/2)F, hybrid mice by the injection of parental (C57BL/6) spleen cells. Intestinal bile salts were measured in runts and littermate controls to assess the physiological significance of the previously described ileal lesion. Bile salt depletion is an early manifestation of this type of graft-versus-host reaction, occurring as soon as 5 days following injection. Loss of bile salts into the colon and attenuation of the bile salt pool may contribute to the clinical syndrome.