An analysis is made of parasite counts made on the first day of treatment in a series of 750 cases of acute subtertian malaria. The analysis affords data of the parasitological intensity of the subtertian infections prevalent in the Kuala Lumpur district of the Malay States during 2 years of normal malarial incidence and severity, and suggests:- 1. (a) That parasite counts made on the first day of treatment are of considerable prognostic reliability. 2. (b) That the counts made during the course of treatment are a useful guide to progress but are open to occasional fallacy. 3. (c) That the extreme limit of tolerance of Asiatic adults for the local strains of P. falciparum is probably in the region of three quarters of a million parasites per c.mm. of peripheral blood.