SUMMARY Repeated in vitro studies of lymphocyte transformation and leucocyte migration were made in 16 renal allograft recipients.Phytohemagglutinin (PHA) was used in the lymphocyte transformation observations and donor spleen and donor kidney cells were used in the leucocyte migration cultures. There was a correlation between the transformation and migration responses and both altered with the changing immunological state of the subject as judged by the functional behaviour of the graft. Immunosuppression rapidly suppressed the transformation response and reversed inhibition of migration that occurred with rejection, the migration index returning to near unity. The pattern of the migration-inhibition response was similar to that obtained when normal subjects are immunised with Brucella antigen, although it developed over a longer interval. Representative data demonstrating these changes are presented.