The in vivo mRNA levels for 16 granule proteins during neutrophil differentiation were determined to address the question of whether the synthesis of granule proteins is regulated individually or blockwise. RNA was extracted from peripheral blood granulocytes and three different populations of neutrophil precursors isolated from human bone marrow by Percoll density centrifugation. The mRNA levels in relation to the maturation of the cells were determined by Northern blot for the 12 matrix proteins myeloperoxidase, proteinase-3, elastase, defensin, lactoferrin, NGAL, hCAP-18, transcobalamin-I, SGP28, gelatinase, lysozyme, and serglycin and the 4 membrane proteins CD68, CD11b, N-formyl-methionyl-leucyl-phenylalanine receptor, and CD35. This panel of transcripts ensured that markers for all exocytosable organelles of the neutrophil were included in the study. A highly differentiated distribution of mRNAs for granule proteins was demonstrated that can explain the heterogeneity of the intracellular storage granules and secretory vesicles of the neutrophil. Furthermore, the individual distribution of these transcripts provides the basis for a more detailed assessment of neutrophil maturation than that obtained by morphological studies or the use of a single marker protein for azurophil, specific, and gelatinase granules. J. Leukoc. Biol. 66: 989–995; 1999.