In order to investigate further the peculiarly limited response of the transplanted pig liver to rejection, this study investigated serial mixed lymphocyte culture (MLC) responses and suppression of phytohaemagglutinin (PHA)-stimulated lymphocytes by serum from transplanted animals. The responses of lymphocytes from liver allografted pigs were compared with those from sham-operated, liver-autografted or kidney auto- or allografted animals for up to 22 weeks postoperatively. Lymphocytes from operated animals were added to cultures of 2 ‘universal’ donors which were not anaesthetised at any time and which remained lymphocyte donors for the period of each operated pig’s survival. After liver allograft, a sustained stimulation of MLC responses was noted which contrasted with the minimal random stimulatory activity seen in the other groups. The response was not seen in lymphocytes from animals subjected to liver autograft. There was a surprising variability in the week-to-week MLC tests of individual animals for which no cause could be found. The serum from liver auto- and allografted pigs had suppressive action on the PHA stimulation of lymphocytes. There was no correlation between the length of survival and the intensity of the pre-operative MLC response. Whilst the biochemical data of liver allografted pigs showed evidence of ongoing hepatocyte damage, the histological changes were less obvious especially in the long-surviving group. This study highlights again the ability of unimmunosuppressed pigs to accept liver allograft and seems to associate this with sustained hepatocyte damage and stimulation of th