contrasuppression by W adherent T8 cells (Lehner et al. 1984). Antibodies to human class II molecules are similarly inhibiting. Whether these I-J like molecules are analogous to their murine counterpart is unknown. It is possible that they act to focus antigen for recognition by the T4-positive helper cells. Despite the enormous progress being made concerning the nature ofT cell receptors, a number of fundamental questions remain unanswered. For example, are the antigen-recognition products on T cell factors similar to the V region products used for the Tr? The finding that factors bind to nominal antigen alone whilst the functional cells, at least for Th and Tc, recognize neither antigen nor histocompatibility molecules in isolation seems to argue against this. It may be that factors make use of different V region sequences of the Tr genes. The question of what genes form the basis of specificity for Ts or contrasuppressor cells is unresolved, as are which parts of the Tra or fi chains bind to antigen or MHC product. New techniques, discussed by Dr Hood, which enable rapid automated sequencing of genes by fluorescence and protein sequencing from as little as one femtomole of material should keep this research moving at a dramatic pace.