Various kinds of bone cements mixed with antibiotics have been widely used for treatment and prevention of infections, especially combined with bone defects. But bone cements have some disadvantages in that they are rather expensive and capable of incurring some complications such ss fibrosis of surrounding soft tissue or local erosion of bone, and furthermore, they must be removed after infection is controlled for bone graft to fill up the defects because they do not have properties of osteoconduction or osteoinduction. So we have come to the idea of using theoretically advantageous plaster of paris to take place of bone cements, and performed in-vitro experiment upon the effectiveness of plaster of paris pellets containing antibiotics, then obtained good results comparable to those from many experimental studies using bone cements. By this, we think that it would be possible in the future to treat and prevent infections supervening bone defects with plaster of paris mixed up with antibiotics. The results are as follows : 1. Sustained release of large amount of antibiotics from plaster of paris enough to exert inhibitory activity on abcterial growth, and there was a direct relationship between concentrations of eluted antibiotics and diameters of inhibitory zones. 2. Tobramycin was capable of inhibiting the growth of S. aureus, E. coli, and Ps. aeruginosa for 7 days at maximum. 3. Cephradine inhibited the growth of S. aureus and E. coli but did not affect that of Ps. aeruginosa. 4. The maximum antibacterial activity wss reached within first 24 hours from the start of elution of antibiotics from plaster of paris.