Serial examinations of the white cells in the blood and serial determinations of serum glutamic oxaloacetic transaminase (GOT) and serum lactic dehydrogenase (LDH) isoenzymes have been performed in 36 patients with seropositive infectious mononucleosis. The first blood sample taken after the onset of the disease showed an increased number of mononuclear cells in the blood in 90% of the cases, elevated GOT level in 70% and elevated LDH level in 100%. Mainly isoenzymes I, II and III accounted for the elevated LDH values, while only 35% of the patients had increased LDH V (liver fraction). The number of mononuclear cells in the blood was normalized in all patients after 5 weeks and GOT normalized after 10 weeks. On the other hand, half of the patients had increased LDH, particularly isoenzymes I, II, and III, 4 months after the onset of the disease. The results indicate that the liver involvement probably is slight and of short duration, while the proliferation of lymphoid cells in the tissues, probably responsible for the main part of the LDH increase, continues for a considerably longer period.