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The haematopoietic microenvironment is believed to play an important role in controlling the haematopoietic process. Ultrastructural studies have shown that the haematopoietic stroma is composed of cellular as well as extracellular components. Relatively little is known about the distribution of the different stromal components in the bone marrow. The knowledge on the bone marrow microenvironment is mainly based on studies in which in vitro long-term bone marrow cultures have been used. Although this culture system offers a unique possibility to study haematopoietic in vitro, it does not fully represent the complexity of intact bone marrow. In the present study we describe the immunohistochemical distribution of different cellular and extracellular stromal components in frozen sections of rat bone marrow as well as in long-term bone marrow cultures, in order to compare the haematopoietic microenvironment used in in vitro studies in the in situ situation. We found that in situ a specific compartmentalization of stromal components exists in the bone marrow. Under culture conditions however, most stromal components are indeed present but the architecture present in the in situ situation had almost completely disappeared. The interaction between the different stromal elements was studied in the long-term bone marrow cultures. It appeared that under the chosen conditions, nodules were formed with a core of reticular cells and extracellular matrix. In close contact with this core immature macrophages appeared to proliferate and differentiate into mature, non-dividing macrophages.