1. Regulation of platelet-derived growth factor (PDGF)-A and -B gene expression was studied using resting monocytes, in vitro matured monocytes and alveolar macrophages. 2. Resting monocytes constitutively transcribed both PDGF-A and -B genes. When monocytes matured to macrophages in vitro, the transcription rates of both genes increased markedly. 3. Consistent with the transcription rates, resting monocytes constitutively expressed both PDGF-A and -B mRNAs. Interestingly, the PDGF-B mRNA levels increased markedly after the maturation, whereas the PDGF-A mRNA did not change. 4. Alveolar macrophages constitutively transcribed both PDGF-A and -B genes at almost the same rates. However, these cells contained 5-fold more PDGF-B mRNA than PDGF-A mRNA. 5. Immunohistochemical study using anti-PDGF-AA and anti-PDGF-BB antibodies suggested that PDGF-BB homodimers were more abundant than -AA homodimers or -AB heterodimers in alveolar macrophages and in vitro matured monocytes. 6. Together these observations indicate that in vitro matured monocytes and alveolar macrophages preferentially express PDGF-B mRNA and produce PDGF-BB homodimers, despite the equal transcription rates for both PDGF-A and -B genes.