To determine a possible influence of two different hormonal contraceptives on bacterial microflora of gingival sulcus, subgingival plaque samples of 29 healthy women aged between 20 and 32 years were investigated bacteriologically before subjects took a contraceptive and 10 and 20 days after subjects started the medication. In 14 women, and oral contraceptive containing 0.02 mg ethinyl estradiol and 0.15 mg desogestrel (preparation A) was used, and 15 women took a contraceptive containing 0.03 mg ethinyl estradiol and 2.00 mg dienogest (preparation B) daily over 21 days. There were no changes in clinical parameters of the teeth investigated during 3 weeks of the study. The periodontopathogenic bacteria Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans were never detected throughout the study. On the other hand, the periodontopathogenic species Prevotella intermedia was found in plaque samples of 22 women. The content of this microorganism showed only a little change between the pretreatment period and plaque sampling after 10 days of contraceptive treatment, but a striking increase occurred after 20 days of contraceptive treatment, especially in the preparation A group. In this respect, there was a significant difference between preparations A and B.