Chronic osteomyelitis has always been a clinical dilemma in repect of resistence to treatment and frequent recurrences. Incorporation of antibiotics in acrylic bone cement has been known to treat chronic bony infection and reduce the incidence of recurrence. The present experiment was undertaken to evaluate the extent of diffusion of various antibiotics from cement-antibiotic admixtures in vitro. By diffusion test, the released antibiotics from bone cement were measured and compared in respect of the amount of mixed antibiotics and the surface area of bone cement plugs. Experimental materials were as follows: Surgical simplex P was selected as a bone cement and antibiotics were divided into three groups. (1) aminoglycoside group: gentamycin and amikin, (2) penicillin group: flucloxacillin, (3) cephalosporin group: cefazolin and moxalactam. Bone cement plugs mixed with antibiotics were prepared and placed in test tube contiaining 10ml normal saline. The test tubes were incubated at 37 degree Celcius. Samples of solutions (0.02ml) were taken from the test tube and assessed by standard microbiological assay with the following bacteias: staphylococcus aureus ATCC 25922. pseudomonas aeruginosa ATCC 27853 and E-coli ATCC 25922. The results obtained were as follows: l. All antibiotics showed a similar pattern of diffusion: an initial rapid release of antibiotics was followed by a more gradual diffusion. 2. Minimal 50.8 percent of the antibacterial activity for moxalactam and maximal 70.8 percent for gentamycin was found during the first 6 hours. 3. The amount of antibiotics relased from the bone cement plugs ranged from 5.75 percent for moxalactam to 9.6 percent for gentamycin. 4. The amount of released antibiotics and the duration of antibacteial activity were proportional to the amount of antibiotics mixed in bone cement. 5. The amount of released antibiotics were closely related with surface area of bone cement: For example, Group 1 for GM was 9.2 percent, group 2 was 9.6 percent and group 3 was 11.1 percent.