In order to determine the in vivo effect of phototherapy on the erythrocytes' osmotic fragility, we have tested 10 preterm infants treated by continuous phototherapy, for jaundice of prematurity. Heparinized blood was obtained daily, starting before the initiation of phototherapy and ending 24 h after stopping the treatment. The osmotic fragility was tested in duplicate immediately, after 1 h incubation at 40 degrees C and after 24 h incubation at 4 degrees C in darkness. The erythrocytes of the preterm infant during and at the end of phototherapy showed a normal osmotic fragility pattern. Comparing the consecutive curves in each infant did not show any population of osmotically differing erythrocytes. Heating or cooling the erythrocytes did not alter these results. It is concluded that phototherapy does not increase the in vivo osmotic fragility of erythrocytes in newborn.