Alkaline phosphatase in neutrophils (NAP) is a product of the liver/bone/kidney-type alkaline phosphatase gene, the chromosomal structure of which has recently been analyzed. The gene has two alternative leader sequences resulting in the two types of mRNA (liver- type and bone-type mRNAs), which suggests that the expression of the gene is regulated by independent promoters. To determine the mechanism underlying NAP induction, it is essential to know which type of the mRNAs is dominant in neutrophils. We adopted quantitative polymerase chain reaction method to determine the relative amount of bone-type mRNA in neutrophils. The bone-type mRNA was found to be at least 5 times more than the liver-type mRNA. mRNA of NAP is known to be induced by in vitro treatment of the cells with granulocyte colony-stimulating factor (G-CSF), which is enhanced by a simultaneous addition of retinoic acid. In neutrophils treated with G-CSF, the bone-type mRNA was at least 6 times more than the liver-type mRNA. In neutrophils treated by both G-CSF and retinoic acid, the bone-type mRNA was at least 22 times more than the liver-type mRNA. The results show that the bone-type mRNA is predominantly transcribed in peripheral neutrophils and in neutrophils cultured in vitro.