Abstract The localization of erythropoietin-responsive cells (ERCs) in the mouse spleen was investigated in vitro. A thin fibrous spleen ‘capsule’ obtained by squashing the organ against a steel mesh was cultured with erythropoietin. The heme synthesis rate in the capsule was compared with that in the spleen fragments from the inner or the outer part of the organ. A far greater heme synthesis was found in the ‘capsule’ than in the other parts of the spleen. On the contrary, the heme synthesis rate in the spleen from anemic mice was higher in the outer part than in the capsule. It is suggested that a larger number of ERCs are located in the ‘capsular’ fragments than in the other portions of the organ. The ERCs may have affinity for the fibrous capsular structure when they are in the undifferentiated stage. The capsular tissue may provide a favorable microenvironment for the differentiation of the ERC.