Monoblasts, promonocytes, and macrophages in in vitro cultures of murine bone marrow were studied ultrastructurally, with special attention to peroxidatic activity. Monoblasts show peroxidatic activity in the rough endoplasmic reticulum and nuclear envelope as well as in the granules. The presence of peroxidatic activity in the Golgi apparatus could not be determined. Promonocytes have peroxidase-positive rough endoplasmic reticulum, Golgi apparatus, nuclear envelope, and granules, as previously reported. During culture, cells are formed with peroxidatic activity similar to that of monocytes or exudate macrophages (positive granules; negative Golgi apparatus, RER, and nuclear envelope); we call these cells early macrophages. In addition, transitional macrophages with both positive granules and positive RER, nuclear envelope, negative Golgi apparatus (as in exudate- resident macrophages in vivo), and mature macrophages with peroxidatic activity only in the RER and nuclear envelope (as in resident macrophages in vivo) were found. A considerable number of cells without detectable peroxidatic activity were also encountered. Our finding that macrophages with the peroxidatic pattern of monocytes (early macrophages), exudate-resident macrophages (transitional macrophages), and resident macrophages (mature macrophages), develop in vitro from proliferating precursor cells deriving from the bone marrow, demonstrates once again that resident macrophages in tissues originate from precursor cells in the bone marrow. Therefore, this conclusion can no longer be challenged on the basis of a cytochemical difference between monocytes and exudate macrophages on the one hand and resident macrophages on the other.