BACKGROUND The shortage of pediatric heart donors often necessitates considerable travel time and, as a result, prolonged donor heart ischemia. This excessive hypothermic storage may contribute markedly to myocardial dysfunction in the recipient. METHODS We investigated the role of leukocyte-endothelial interactions in this dysfunction in an isolated, immature (mean age, 11.8 +/- 1.6 days) swine heart model using a monoclonal antibody against a leukocyte adhesion molecule. We studied a total of 20 hearts subjected to 6 hours of cardioplegic arrest at 4 degrees C. Group M1/70 (n = 6) received at reperfusion 15 micrograms/mL of a monoclonal antibody F(ab')2 fragment to CD11b, the alpha-subunit of the leukocyte adhesion molecule Mac-1. Group MB10.6 (n = 8) received 15 micrograms/mL of the swine unreactive F(ab')2 MB10.6, and the third group received saline vehicle. RESULTS Administration of M1/70 resulted in improved postischemic recovery of ventricular function compared with the two control groups (p < 0.05). CONCLUSIONS These data implicate leukocyte-endothelial interactions mediated by the leukocyte adhesion molecule CD11b in myocardial dysfunction after long-term hypothermic ischemia. Specific antiadhesion strategies such as this may safely extend storage time for pediatric donor hearts.