The purpose of the present study was to test the plaque-removing efficacy of 4 different toothbrushes in relation to duration of toothbrushing. The brushes studied were a manual toothbrush (M), a conventional electric toothbrush--the Blend-a-Dent (BL), the Interplak (IP) and the Braun Plak Control (BPC). For this study, 20 subjects were selected. The study was divided into 5 experiments which differed only in respect to the brushing time. The available time in experiment 1 through 5 was 7.5, 15, 30, 45, 90 s per quadrant respectively. Prior to each experiment, all subjects were asked to abstain from oral hygiene procedures for at least 24 h. The efficacy of toothbrushing in each experiment was studied while one investigator (MAL) brushed the teeth of the subjects. In each subject, all 4 brushes were tested. Each brush was assigned to a quadrant in a random order. No toothpaste was used throughout this study. The amount of dental plaque was evaluated before and after brushing by means of the Silness & Löe plaque index at 6 sites around each tooth investigated. The results show an increase in efficacy for all brushes varying from 7.5 s per quadrant to 90 s per quadrant (mean plaque reduction in terms of percentage 7.5 s-90 s: M = 40%-75%, BL = 45%-82%, BPC = 51%-94%, IP = 64%-92%). The IP removed significantly more plaque than the other 3 after 7.5 s of brushing. From 15 s through 90 s, the IP and BPC were equally effective.(ABSTRACT TRUNCATED AT 250 WORDS)