The levels of DNA polymerase alpha, delta, epsilon were examined in the neonatal mouse forebrains and spleens. The levels of DNA polymerase alpha were determined by the difference of polymerase activity in the absence and the presence of alpha specific inhibitor, BuPdGTP, or its monoclonal antibody. The levels of DNA polymerase delta were determined in H.A fractions after separating it from the other two enzymes. The levels of DNA polymerase epsilon were identified in H.A fractions by the use of alpha-monoclonal antibody or BuPdGTP. Results showed that in the mouse forebrain DNA polymerase alpha, delta, epsilon activities are the highest before birth, decline sharply following birth and are very low on the 8th day and hardly detectable on the 17th day; as for the mouse spleen, however, DNA polymerase alpha, delta, epsilon activities are the lowest at birth, increase rapidly after birth and reach their maxima on the 8th day and then decline gradually but remain in higher levels. These results not only prove that DNA polymerase alpha and delta take part in cell DNA replication but also suggest that DNA polymerase epsilon is involved in DNA replication.