DISEASE CHARACTERISTICS: Histologically confirmed stage B, C, or D adenocarcinoma of the prostate not amenable to surgery

PATIENT CHARACTERISTICS: Age: 40 and over Performance status: ECOG 0-2 Life expectancy: At least 6 months Hematopoietic: WBC at least 3,500/mm3

Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 2.0 mg/dL Transaminases less than 4 times upper limit of normal Renal:

Creatinine less than 2.0 mg/dL Cardiovascular: No New York Heart Association class III or IV heart disease Other: No significant abnormalities of internal organs, neurologic status, or biochemical levels No history of seizures No concurrent serious illness No acute infection requiring antibiotic therapy except chronic urinary tract infection No other prior or concurrent malignancy within the past 2 years except nonmelanoma carcinoma of the skin No history of substance abuse or addiction No alcohol intake greater than 2 drinks/day or 14 drinks/week No diet containing more than 20 mg of genistein/day No known soy intolerance No prior breast cancer

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: Not specified Endocrine therapy: At least 3 months since prior hormonal therapy OR No concurrent estrogen Other concurrent hormonal therapy allowed if stable regimen for more than 3 months Radiotherapy: At least 4 weeks since prior radiotherapy and recovered Surgery: See Disease Characteristics At least 3 weeks since prior surgery and recovered Other: At least 6 weeks since prior antibiotics No concurrent genotoxicity therapy