To investigate the cellular origins of cathepsin D (CD) in inflammatory lesions, the CD content of lymphocyte subsets, monocytes, and macrophages were compared. Human monocytes, B lymphocytes, CD4+ T lymphocytes, and CD8+ T lymphocytes were separated from peripheral blood of normal donors. CD content was 0.13 +/- .01 micrograms equivalents of CD per million cells and significant differences between different cell types were not found. To determine the CD content of macrophages, differentiation of peripheral blood monocytes was induced by either in vitro culture or treatment with 4 beta-phorbol-12-myristate-13-acetate (PMA). Macrophages induced by five-day culture contained four times more CD than unstimulated monocytes, and macrophages induced by 18-h treatment with 20 mg/ml 4 beta-PMA contained nine times more CD than monocytes treated with 4 alpha-PMA, an inactive stereoisomer of 4 beta-PMA. These results suggest that macrophages are one of the enriched sources of CD in inflammatory lesions.