The effects of 0.1% and 0.2% delmopinol mouthwashes on supragingival plaque flora were investigated in a 6-month home-use study. 141 subjects were studied from whom plaque was collected at baseline, 12, 24, and 36 weeks. Overall, there were no consistent effects on microscopic or total counts. However, there was a significant reduction in the proportion of dextran-producing streptococci in the active groups compared to the control group throughout treatment. There was no colonisation by Candida or Gram-negative aerobic bacilli in the active groups nor was there any decrease in susceptibility to delmopinol. Delmopinol appears to mediate its anti-plaque effect without causing a major shift in bacterial populations, although dextran-producing bacteria appear to be affected, which may have relevance to this agent's mode of action.