Using in vitro lymphocyte tissue culture screening tests human heart transplant recipients during the academic year 1973/1974 were evaluated and an attempt made to predict their post-operative rejection course. The tests involved screening for serum depressive factors and screening for the recipient's lymphocyte reactivity. A scoring system was devised whereby the laboratory results could be converted to a predicted rejection score. Of the nine patients studied, eight are still doing well. Three recipient's who had a high postoperative rejection score as determined by an independent clinical team, were correctly identified pre-operatively and one of them had to be retransplanted. These screening tests may be helpful in the future in the selection of recipients.