Phototherapy to prevent hyperbilirubinemia in the premature infant was tested with 47 light-treated and 51 control infants with birth weights of 1,250 to 1,500 gm (2 lb 10 oz to 3 lb 4 oz). All infants had a negative Coombs' test. Treated infants were continuously nursed under a light chamber for 24 hours to 6 days of age. Bilirubin equaled or exceeded 15 mg/100 cc in 25.5% of the controls and none of the light-treated infants. Treatment was as effective in Negro and Mexican-American infants as in white infants. The only difference noted in clinical behavior was the appearance of green stools in light-treated infants. Evaluation of growth and development of these infants will be necessary to establish the benefits and safety of phototherapy.