One single injection of estradiol to male guinea pigs resulted in the appearance of Kurloff cells in the thymus and spleen in a maximal number after 2–3 weeks. In both organs, Kurloff cells of three categories were observed with different size and number of the inclusion(s). In the thymus, cells with small and medium-sized inclusions were present almost exclusively among low-density thymocytes. Cells with one large inclusion were initially present among low-density cells, but with time an increasing proportion were found among high-density thymocytes. This indicates that the different categories of Kurloff cells represent maturational stages and follow the normal differentiation step of thymocytes from low to high density. According to electronic cell volume distribution analysis, estradiol treatment was associated with a shift in cell volume towards larger cells, and this shift was correlated in time with the appearance of Kurloff cells with large inclusions. In thymus sections there was evidence for a massive and selective migration of Kurloff cells via interlobular lymph vessels. No difference in localization of Kurloff cells was noted at various times after estradiol treatment. Attempts to induce Kurloff cell formation from bone marrow, spleen or thymus cells during 2 weeks in vitro were unsuccessful, also when culturing the cells in serum from estradiol-treated animals. This negative result, the long lag phase of estradiol-induced Kurloff cell formation in vivo, and the reported lack of estradiol receptor in Kurloff cells indicate that the Kurloff cell induction by estradiol may be indirec
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