Prophylactic treatment of couples with oral penicillin has been recommended as a means of eradicating GBS from the parturient female. In 1977 and 1978 this hypothesis was tested in an investigation of couples from a middle and upper socioeconomic group in Houston, Texas. A group of 40 women, known to be colonized with GBS during the third trimester of pregnancy, and their husbands were treated simultaneously with oral penicillin. Patients were recultured 3 weeks following completion of therapy and at the time of delivery. At the time of delivery 67% of the women remained colonized with GBS. This percentage does not differ substantially from that obtained in a series of untreated colonized women. This study demonstrates that oral penicillin treatment of couples is not an effective means of reducing maternal colonization at the time of delivery.