This study was undertaken to measure the diffusibility and antimicrobial effectiveness of the medication used in clinical practice. To study the diffusibility of the root canal medicament, loss of formocresol and camphorated phenol from a cotton pellet after insertion into the pulp chamber of 260 molars prepared as routine endodontic treatment was measured. Measurement was done for the one time insertions and for the reinsertions using ultraviolet spectrophotometer. Antibacterial effectiveness against three microoganisms, Staphylococcus aureus, Staphylococcus epidermidis and a-hemolytic streptococcus, on the blood agar plate was observed by measurement of the inhibition zone with. various amount of medicaments. The following results were observed. 1. Nearly all of the medication were lost in the first day after insertion and the residual amount of camphorated phenol was greater than that of formocresol. 2. Residual amounts of medication in the reinsertion group were greater than that of the one time insertion group. 3. Within the pulp chamber diffusibility of formocresol was greater than that of camphorated phenol. 4. The amount of formocresol diffusing out from the tooth was greater than that of camphorated phenol. 5. Antibacterial effectiveness was observed from the residual amount of formocresol in the reinsertion group and in other groups no antibacterial effectiveness was observed.