Immunohistochemical and ultrastructural studies have indicated the existence of a distinct "fetal macrophage" type, differing from monocyte-derived macrophages. In order to characterize macrophages of different ontogenetic origins on the molecular level, we examined their surface-marker and marker-gene expression patterns. We found that macrophages derived from chicken embryos express the lysozyme gene at significantly lower levels than macrophages derived from adult chicken. The same was observed when expression of the chicken lysozyme gene was analyzed in transgenic mice. In three independent mouse lines, mature macrophages derived from embryonic or fetal hematopoietic tissues expressed the transgene at drastically lower levels than macrophages derived from the bone marrow, spleen, or peritoneal cavity of adult mice. Macrophages obtained by in vitro differentiation of mouse embryonic stem cells (a process resembling early embryonic hematopoiesis) displayed the embryo-specific low transgene expression level. Experiments determining the developmental potential of myeloid precursors in culture and immunophenotypic analyses revealed differences between embryo-derived and adult myeloid progenitor populations. In summary, our results provide further evidence for the existence of dissimilar embryonic/fetal and adult macrophage types and describe the first molecular marker for their distinction.