The private sector plays a major role in the delivery of health care in South Africa. Over the past two decades, the quality and equity of such provision has been questioned internationally. A study was conducted in Gauteng Province to explore these issues, using care for sexually transmitted disease (STD) as a case study. Private general practitioners (GPs) were interviewed by telephone. Each was presented with a set of STD syndromes and requested to describe how s/he would manage the patient, first if the patient was insured, then secondly if the patient was paying cash (uninsured). Reported prescriptions were costed and assessed for effectiveness against main causative pathogens using local standard clinical guidelines. Knowledge of recent developments in STD syndromic management and effectiveness of prescribed drugs was poor, especially for genital ulcer and pelvic inflammatory disease, and less than half the prescriptions overall were judged as effective. Although the effectiveness of prescriptions for insured and uninsured patients were similar, for most syndromes uninsured patients were offered significantly cheaper and less convenient antibiotic regimens. Effective regimens were also significantly more expensive than ineffective regimens. The results suggest that GPs' perceptions of patients' willingness or ability to pay for drugs have a bearing on quality of care. The paper concludes that STD patients who present to GPs are often offered poor quality of care, and the choice of inconvenient antibiotics impacts disproportionately on the poor. Improvements in the quality and equity of GP care will require interventions that address the factors that determine their behaviour.