The granule proteins are among the most abundant and characteristic proteins of myeloid cells. They are essential for the antimicrobial activity of these cells and they provide important markers for the differentiation stage of the myeloid series and for the diagnosis of myeloid leukemias. In acute promyelocytic leukemia (APL) there is high production of myeloperoxidase, and its cytochemical detection as well as the t(15;17) chromosomal translocation are important markers in the diagnosis of this acute myelogenous disease. The expression of other granule protein genes in APL has not been systematically determined. We have used the reverse transcriptase-polymerase chain reaction (RT-PCR) method to determine the pattern of expression of granule protein genes at the mRNA level in APL cells. We have examined the expression of the primary granule proteins defensin, myeloperoxidase, elastase, and cathepsin G; the secondary granule proteins lactoferrin, collagenase, and transcobalamin; as well as lysozyme, a protein reportedly found in both primary and secondary granules. mRNAs for all of these granule proteins were present in normal bone marrow mononuclear cells. We found that APL cells from three patients contain, in addition to myeloperoxidase mRNA, mRNAs for elastase, cathepsin G, and lysozyme. One patient had faint but detectable lactoferrin mRNA signal, but collagenase and transcobalamin mRNAs were not detectable in this patient. Defensin mRNA was found in one of the three APL patients, and all the primary granule protein mRNAs measured were found to be expressed in the APL cell line NB4. None of the secondary granule protein mRNAs measured were detectable in NB4 cells. After treatment with retinoic acid (RA), which induces neutrophil maturation of these cells, weak induction of lactoferrin and collagenase but not transcobalamin was observed. However, in view of the weak transcobalamin signal observed in normal bone marrow, the absence of transcobalamin in RA-induced NB4 cells must be interpreted with caution. Interestingly, elastase and cathepsin G mRNA disappeared after RA induction, whereas defensin and myeloperoxidase mRNAs remained present. These findings indicate that granule protein mRNAs are regulated separately and differently, and that only minimal expression of secondary granule protein genes can occur in APL cells.