Sera from 196 patients were collected before and after cardiac surgery to measure antibodies against heart tissue and against actin and myosin. In the post-operative period antibodies were found in 87 patients (44%) producing a cross-striated fluorescence pattern in heart tissue. Antibodies against the major contractile proteins were found in 91 patients (46%), anti-actin antibodies in 49 patients (25%) and anti-myosin antibodies in 65 patients (33%). We found a significant correlation (P less than 0.0001) between the antibodies producing a cross-striated fluorescence pattern and antibodies recognizing contractile proteins. These results suggest that contractile proteins evoke an immune response after cardiac surgery and that this response may compromise cardiac function.