The transition in frequency of Y chromatin in males during the neonatal period.

The frequency of Y chromatin, visualized as fluorescent bodies in cell nuclei from lymphocytes in blood smears, was significantly less in newborn males than in three-month-old male infants and adults. The frequency of Y chromatin-positive cells on day 0 was 36.16 +/- 9.11% and then increased daily. At one month after birth the frequency was 55.07 +/- 9.29%, which was not significantly different from that in adult males (57.08 +/- 5.97%).