Passive enhancement was attempted with F (ab') 2-fragments in six patients receiving their first and in one patient the second one haplotype mis-matched kidney graft. Administration of enhancing antibody in the first graft patients seemed safe, the group was however too small to determine whether the enhanced patients did better than those conventionally treated. The only pre-sensitized patient who received his second transplant rejected it during the first week, possibly due to a deleterious effect of the F (ab') 2-fragments.