The author studied the effect of weak direct current on bacterial growth in vitro and in vivo. In vitro, electric current was applied 20 or 100 micro A/cm2 of direct current using electrode of carbon, silver or platinum. Its inhibitory effect was observed on the growth curve of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa in the anode bath respectively. A silk thread that adsorbed Staphylococcus aureus was inserted into the intramedullary space of the tibia of Wistar rats to induce osteomyelitis. Silver electrodes were placed to apply 100 microA/cm2 of direct current for two weeks. The therapeutic effect was then evaluated in terms of X-ray findings, histological findings and changes in the viable count of Staphylococcus aureus in the intramedullary space of the tibia. It was found that application of electric current using a silver electrode was most effective for the inhibition of bacterial growth both in vivo and in vitro, and seemed to be clinically useful for treating osteomyelitis.