C reactive protein (CRP) concentrations were measured serially in 38 patients after cardiac transplantation. Three of 28 patients (11%) had raised values before transplantation. After transplantation, most patients showed rises in CRP concentrations associated with transplant surgery which became normal by day 7. Thereafter, 75/274 samples (28%) from 18/38 patients (47%) had raised CRP values. Most of the rises in CRP concentration were associated with infection (78%), which in most cases was bacterial. No correlation was found between CRP values and acute cardiac rejection assessed histologically on serial endomyocardial biopsy.