session was introduced by one speaker, followed by free discussion. The intro­ ductory lectures are included in the proceedings. The participants summarized their contributions to the discussion in written form. In addi tion to the closed sessions, there was one open session at the Karolinska Institute with lectures by Drs. L. Hood, C. Milstein, D. Baltimore, J. Klein and B. Benacerraf, which are not included in these procee din gs. The symposium was sponsored by the Nobel Foundation and its Nobel Symposium Committee through grants from the Tercentenary Fund of the Bank of Sweden and the Knut & Alice Wallenberg Foundation. The Swedish Medical Research Council, the Swedish Cancer Society and the Swedish Ministry of Education and Cultural Affairs also made contri­ butions.