This study investigated the ability of foetal thymic epithelium prepared by 24 degrees culture (24 degrees-TE) or treatment with deoxyguanosine (dGuo-TE), to induce tolerance in nude mice. Thymic chimeras were constructed in which the thymic epithelium differed from the host at both major histocompatibility complex (MHC) antigens and multiple minor histocompatibility antigens (mHa), or at mHa only. Peripheral CTL from nude mice receiving dGuo-TE disparate for mHa, or both MHC antigens and mHa, were uniformly tolerant of thymic mHa. CTL from nude mice grafted with 24 degrees-TE or dGuo-TE were tolerant of host MHC antigens, but the two treatments differed in the efficiency with which they induced tolerance to thymic MHC antigens. CTL responses specific for thymic MHC antigens could be generated in vitro from dGuo-TE grafted mice but not from those receiving 24 degrees-TE. The addition of concanavalin A (Con A) supernatant had no effect on the CTL tolerance observed in 24 degrees-TE grafted mice, suggesting that the lack of CTL responses was not due to tolerance in MHC class II restricted ‘helper’ cells. However, CTL responses against the thymic MHC antigens of dGuo-TE grafted mice displayed high sensitivity to blocking by anti-CD8 antibodies, indicating that these CTL were of low affinity. These results suggest that 24 degrees-TE induces tolerance in most thymic MHC-specific CTL precursors, whereas dGuo-TE induces tolerance only in CTL with high affinity for thymic MHC antigens. Therefore, 24 degrees-TE and dGuo-TE are both capable of inducing CTL tolerance, consistent with the previously reported acceptance of thymic donor-type skin grafts by nude recipients of dGuo-TE treatment. We conclude that MHC class I molecules on thymic epithelium play a role in negative selection of the developing T-cell repertoire.