During a clinical trial for evaluating the antiplaque effect of two flavoured chlorhexidine rinses, the observation was made that bleeding after gentle massage of the gingival margin occurred more often after chemical than after mechanical oral hygiene measures. In order to ascertain the validity of this unexpected observation, the same dental students participated in a repetition of the trial 18 months later. The results of the two trials indicate that gingival bleeding after gentle massage of the margin with the side of a periodontal probe actually occurs more frequently after rinsing twice daily with a 0.2% aqueous chlorhexidine solution for 1 week than after meticulous mechanical oral hygiene measures during an equally long time period. The average frequency of bleeding, in per cent of all examined gingival units, ranged from 1.3% after mechanical cleaning of the teeth to 5.4% after rinsing with chlorhexidine for 1 week. Neither the frequency of bleeding nor the difference between mechanical and chemical plaque control were considered to be of clinical significance. Further microbiological and histological studies are being conducted in an attempt to clarify the reason for the observed bleeding tendency.