Time-course studies in ten allograft recipients showed a significant reduction in the percentage of T cells in the immediate post-transplantation period. This was accompanied by an increase in the percentage of both B and 'null' cells. T, B and 'null' cell percentages reverted to normal by the 6th month after transplantation at a time when patients had achieved stable renal function and were receiving only maintenance immunosuppressive therapy. Similar trends were observed with regard to absolute number of T, B and null cells. Both pre- and post-transplant plasma samples failed to inhibit T-rosette formation. Transient B rosette inhibitory activity was, however, observed in the plasma samples of two patients but did not appear to correlate with the mixed leucocyte culture blocking factor activity (MLC-BFA) of these plasma samples. In the group of patients studied there was no apparent association between percent or absoulte T, B and 'null' cell determinations and the onset of rejection crises.