Phase platelet counts in a large series of premature infants showed mean levels of 220,000/mm3, 260,000/mm3 and 309,000/mm3 during the first 2 days and in the second and in the fourth week respectively. An additional confirmatory series of infants with weights of 1,500 gm or less showed comparable values. These were followed during the first 2 days, the sixth or seventh day, the tenth or eleventh day, the fourteenth or fifteenth day, and the twenty-eighth to thirtieth day. Mean values were 203,000/mm3, 255,000/mm3, 272,000/mm3, 309,000/mm3, and 354,000/mm3, respectively. A steady rise was generally observed during the first month of life irrespective of birth weight. In the first 2 days of life, platelet values under 150,000/mm3 are more common in premature infants than in full-term babies. However, counts under 100,000/mm3 at any time are unusual and suggest a search for pathological factors. Figures under 50,000/mm3 should be considered frankly abnormal under any circumstances.