The appearance of immunologically competent cells in the organ-cultured mouse fetal thymic rudiment has been investigated. Fetal thymuses removed at 14 d of gestation and cultured for 7-21 d were assayed for their content of cytolytic T lymphocyte precursors (CTL-P) directed against H-2d alloantigens. Whereas CTL-P were undetectable within fetal thymus until 18-19 d of gestation, their frequency in the organ-cultured fetal thymus was similar to, or greater than that found in the normal adult thymus. This direct demonstration of the appearance of alloreactive CTL-P in a closed in vitro system should provide an accessible model for the investigation of interactions between developing T cells and the thymic microenvironment.

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