A marked increase in the activity of the neutrophil alkaline phosphate enzyme (NLAPA) was observed in bacterial tonsillitis whereas in infectious mononucleosis the amount of enzyme in the leucocytes was either decreased or normal. NLAPA of patients with tonsillitis decreased in proportion to the receding disease; in infectious mononucleosis the enzyme increased during convalescence. In 16 of 20 patients with infectious mononucleosis, NLAPA increased after corticotherapy. The value of the enzyme in patients with infectious mononucleosis was much higher after superinfections caused by a different infective agent than after corticotherapy. The studies demonstrate the value of NLAPA examinations in the early diagnosis of infectious mononucleosis, carried out during the acute febrile state and especially during the first days of the illness when all other laboratory examinations may still give negative results.

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