Dental plaque maturation in both caries-resistant (CR) and caries-susceptible (CS) subjects was accompanied by a decrease in total lipid content but was more pronounced with CR subjects; their plaque after 72 h contained 44.6 per cent less lipids than that of CS. At each stage of maturation, differences were also found in the proportion of lipid classes. In 24 h-plaque, the lipids of CR-group, although containing amounts of glycolipids and phospholipids similar to those of CS, contained 1.6 times less neutral lipids. The 48 h-plaque of CS subjects showed 2.5 times more neutral lipids, 1.2 times more phospholipids and 1.4 times less glycolipids. The 72 h-CR plaque contained 1.9 times less neutral lipids, 1.6 times more glycolipids and 1.3 times more phospholipids than CS-plaque. Among the neutral lipids in both groups, the changes with maturation were associated with decrease in free fatty acids and increase in triglycerides, cholesterol and cholesteryl esters. Phospholipids of CR-plaque contained less phosphatidylcholine and more phosphatidylethanolamine than CS-plaque. With plaque maturation both groups showed decreases in the glycolipids characteristic of saliva and an increase in the content of bacterial type of glycolipids. Plaque maturation is evidently accompanied by dynamic changes in lipids reflecting a shift from salivary to bacterial contribution.