Heterogeneity of progesterone receptor (PR) expression in MCF-7 cells is generally attributed to the coexistence of several sublines, each possessing different stages of differentiation. One hypothesis is that the variation of PR distribution relates to the genotype cell heritage and cell cycle phases. The aim of this study was to demonstrate the implication of cell subclones in PR heterogeneity. MCF-7 cell line subclones were obtained initially by the limit dilution method on microscopic slides. On these slides PR was assessed by immunofluorescence. 20 of the subclones were PR-negative, 10 were positive with varying degrees of PR expression. As these cell populations arose from a single cell, they can be considered as monoclonal. These results show that PR heterogeneity (positive vs. negative clones) is based on a clonal origin and could be genotypically explained. In a second experiment four PR-positive MCF-7 cell subclones were maintained in continuous culture and studied. On each one a triple fluorescent staining (PR, Ki-67 antigen and DNA) was performed and the reactions were quantified by videofluoro microscopy. These results demonstrated that a relation between cell PR content and cell cycle stages exists in these four subclones. Cells in G₀ express only little PR; PR level increases during the S phase to reach a maximum in the G2 phase; after mitosis PR level decreases with cell division and degradation may occur in G1: PR level reaches a minimum in late G1 and in the early S phase. The doubling times of the different MCF-7 subclones shows that those that are rapidly cycling were preferentially PR-positive, whereas slowly cycling MCF-7 subclones were PR-negative. We conclude that in MCF-7 cells some subclones are able or not able to synthesize PR; PR content is directly dependent on cell cycle phase and population doubling time.