We have analysed the bacteraemias occurring before discharge from hospital in patients subjected to open-heart surgery. The overall incidence was 2-7%. The predominant organisms at first were staphylococci and streptococci, but in the later years of the study almost all positive blood cultures were due to Gram-negative bacilli. Short-term prophylactic antibiotics were used throughout. Eleven of the 12 Gram-positive bacteraemias occurred after the period of antibiotic prophylaxis; eight of the patients had endocarditis and a further three had serious wound infections. Most of the 32 Gram-negative bacillaemias occurred while antibiotics were being given. Some were transient phenomena in well patients; others were associated with terminal states. Infection was a major cause of death in four patients, however, and two patients had endocarditis. Serratia, providence, hafnia, and citrobacter were all identified as well as the more usual escherichia, klebsiella, and pseudomonas. We discuss the possible origins and management of these bacteraemias.