The neonatal immune system is immature and may be affected by Bacillus Calmette-Guerin (BCG) vaccine. We investigated the influence of BCG given at two different ages on the peripheral blood (PB) T-cell subpopulations. Forty full term healthy newborns were randomly chosen. Twenty of them were vaccinated with BCG at birth (group 1) and the remaining at the age of 2 months (group II). The cell analysis were carried out before (pre-BCGI and pre-BCGII), and 2 months after (post-BCGI and post-BCGII) the vaccination. The analysis of the gamma/delta and alpha/beta T-cell receptor (TCR) antigens was done by two-colour flowcytometer. The purified protein derivative (PPD) response was investigated 2 months after vaccination. The results showed that although T-cell (TCR+ cell) counts showed no difference in PB before and after vaccination in both study groups, the total lymphocyte and non-T cell (TCR- cell) populations increased significantly whereas alpha beta T-cell population significantly decreased after vaccination. On the contrary, gamma delta T-cell counts in PB increased significantly 2 months after vaccination in group I but not in group II. Total lymphocyte and non-T cell counts in vaccinated infants at 2 months of age (post-BCGI) were significantly higher than in unvaccinated infants of the same age whereas alpha beta T-cell count in vaccinated infants was significantly low. However, total T-cell and gamma delta T-cell counts showed no difference. PPD positivity was similar in both study groups (61% in group I, 66% in group II). Neither alpha beta T- nor gamma delta T-cell counts were different in PPD positive and PPD negative infants. Our study shows that BCG causes marked quantitative changes in the PB T-cell subpopulations in young infants.