A prospective study was undertaken to investigate the occurrence and possible sources of microbial contamination in association with open meniscectomy of the knee. Sixty patients were randomized into three closed suction drainage regimens of 12, 24 and 48 hours' postoperative suction drainage, respectively. Specimens for bacterial culture were taken from the joint at the time of surgery, from the surgical suction tips, from the subcutaneous part of the drain surface and from the drain-contained fluid at the time of drain removal. The airborne bacterial contamination showed no marked role in this bacterial invasion into the joint, nor did the use of suction increase the risk. The vacuum suction drain, on the other hand, appeared to carry an obvious risk of bacterial invasion, as a parallel of time. In the present comparison this risk was minimal when the drain was removed 12 hours postoperatively.