The recent association of menstruation, tampon use, and staphylococcal infection with toxic shock syndrome led us to study the association of menstruation and catamenial product use with changes in vaginal flora. Cultures of the cervical os were obtained in midcycle and during menstruation from 12 women who used napkins and 40 women who used tampons. Staphylococcus aureus was found during midcycle and menstruation in three women, during menstruation alone in six, and during midcycle alone in none, indicating a significant association of S. aureus with menstruation (p = 0.04). No difference was found in the rate of S. aureus colonization during menstruation in tampon users (18%) and napkin users (17%). In a similar study, cultures were taken for S. aureus and other aerobic bacteria from 58 tampon users and 25 users of sea sponges. Staphylococcal colonization was found to be increased during menstruation in both groups. Among the cultures done during menstruation, those from users of sea sponges were found to have significantly higher colonization rates with S. aureus, Escherichia coli and other Enterobacteriaceae. The association of sea sponges with a high rate of S. aureus colonization suggests that they are not an alternative to tampons for women seeking to decrease the risk of toxic shock syndrome.