PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: At least 1 week since prior tretinoin No prior retinoids (greater than 25,000 IU vitamin A or equivalent for at least 3 months) No other tretinoin during or for 3 months after study Endocrine therapy: No concurrent regular steroids Radiotherapy: Not specified Surgery: See Disease Characteristics Other: No concurrent regular anticoagulant medication No concurrent nutritional supplements other than 2 multivitamins per day