Analysis of blood by polymerase chain reaction (PCR) is a more rapid and sensitive method to detect bacteremia than blood culture. The PCR was performed on blood obtained from patients during blood culture draws and on blood from normal volunteers. Eighty-seven patients provided 125 blood samples for blood culture comparison with PCR. Specific PCR primers for Staphylococcus aureus and Escherichia coli that targeted conserved regions common to gram-positive and gram-negative bacteria were used. Selective stringency conditions identified other gram-positive and gram-negative bacteria. The blood culture agreed with the PCR in 93 of the 125 patient specimens (74%). In 29 of these specimens the PCR was positive yet the blood culture was negative. When clinical information was included with positive blood culture to define sepsis in these patients and their specimens were added to the positive blood cultures the statistical accuracy of PCR was 93 per cent. Only three of the 78 specimens with negative PCR had positive blood cultures. The PCR was negative in all but one of the 50 volunteers. PCR is more sensitive than blood culture, and it can quickly rule out bacteremia.