A study was conducted to determine whether there was any advantage in using a commercial mouthwash over water in a pulsating oral irrigating device on dental plaque growth and maturation. The amount of plaque accmulated on the test teeth of human subjects was measured on days 1,3,5,7 and 10 during the experimental period by using the two-tone disclosing dye solution and the Quigley and Hein scoring method. The results showed the value of using the two-tone disclosing dye in visualizing the growth and maturation of bacterial plaque in situ. There was a marked delay in the rate of plaque maturation by the commerical mouthwash delivered by an irrigating device. There were no statistical differences in plaque accumulation between the control and treatment groups for the 10-days test periods.