We evaluated a semiquantitative culture technic for identifying infection due to intravenous catheters: rolling the catheter segment across blood agar. This method was compared to broth culture. Of 250 catheters studied, 225 (90%) had low-density colonization on semiquantitative culture (less than 15 colonies on the plate) although 49 (19.6%) of these grew some organisms in broth or on the plate. None of these catheters led to septicemia. Twenty-five catheters (10%) grew greater than or equal to 15 colonies by the semiquantitative technic; most gave confluent growth. Septicemia originated from four of these catheters (P = 0.008). Of 37 catheters exposed to bacteremias from distant foci of infection, four yielded matching growth in broth, whereas none were concordant with the blood isolate on semiquantitative culture. Local inflammation was associated with high-density colonization semiquantitative culture (P less than 0.001). The semiquantitative technic distinguishes infection (greater than or equal to 15 colonies) from contamination and is more specific in diagnosis of catheter-related septicemia than culture of the catheter in broth.