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Young mice of the strains A/Y and B10.D2 were repeatedly treated with diethylsulphate (DES) in different doses (36 to 1100 mg/kg). At the age of 9 weeks they were mated to females of the strains A. CA (' A' group) and C57BL/10Eg ('D' group) respectively. 2101 progeny of these matings were tested for histocompatibility by skin grafting. The spontaneous Hmutation rates were 6-96 x 10~4 per gamete in the A group and 9-6 x 10~4 per gamete in D group. In progeny of treated males the H-mutation rates were 0 in A group and 5-79 x 10~3 per gamete in D group, showing apparent effect of paternal DES treatment on mutation frequency in the last group. Two mutations of the H-2 locus were found, which together with the other three H-2 mutations published so far yielded a mutation rate of 5-18 x 10~* per gamete. The mutation rate of the H-2 locus is higher than the expected rate perH-locus, indicating a great genetic complexity of H-2.

Specific suppressor T cells (SSTC), primed in vivo with H-2 antigens, have been shown previously to inhibit DNA synthesis in the one-way, three-cell mixed lymphocyte reaction (MLR) provided that (a) the stimulator cells bear the priming H-2 antigens, and (b) the responder cells possess IC + S regions homologous to those of the SSTC. Anti-B10.A B10.A(2R) SSTC (anti-Dd) and anti-A.AL A.TL SSTC (anti-Kk) are shown here to be able to inhibit the DNA synthesis triggered in MLR, not only by the corresponding antigens, Dd and Kk, respectively, but also by irrelevant, third-party H-2 and Mls products provided that the corresponding and third-party antigens are presented on the same stimulator cell. If stimulator H-2 regions, whose products interact with SSTC and responders, are located on different stimulator cells within the particular MLR, SSTC activity is not elicited. Participation of cytotoxic T lymphocytes in DNA-synthesis suppression is ruled out. Direct contact or location of the inhibited responder cell very close to SSTC is considered to be required for the development of SSTC activity.