A sensitive method for immunocytochemical detection of estrogen receptors using flow cytometry is reported. Using this method, estrogen receptors were detected in several osteoblastic cell lines with established expression of estrogen receptors, and for the first time, estrogen receptors were also demonstrated in murine fibroblasts and in human primary marrow stromal cells. The distribution of estrogen receptors within all cell lines was unimodal. The method enables for studies of estrogen receptors in the cell types which express low to moderate levels of the receptors, for studies of heterogeneity of the receptors' expression and for simultaneous detection of several parameters on a single-cell level.