SummaryA sensitive,in vitromethod has been developed for the study of bacterial adsorption to dental crown and bridge cements. The method employs heavily radiolabelled cells which adhere to the wall in wells made from the cements. Loss of radioactivity from the bacterial suspension during incubation is used to measure the degree of bacterial adherence to the materials.Strains ofStreptococcus sanguisandStreptococcus mutansadhered in greater numbers than a strain ofStreptococcus salivariusto both a zinc phosphate cement and a zinc carboxylate cement. The zinc phosphate cement supported adherence to a greater degree than did the zinc carboxylate cement. Pretreatment of the cement surfaces with human saliva tended to reduce bacterial adherence. Pretreatment of the bacteria with saliva slightly enhanced sorption ofS. sanguisandS. salivarius, but tended to impair sorption ofS. mutansto both types of cements. When both bacteria and cements had been pretreated with saliva, a strong inhibition of the adherence ofS. mutanswas observed.