Antibodies against the P53 tumour suppressor gene product are present in the serum of many cancer patients, but with varying frequencies ranging from 0 to 30%. Approximately 15-20% of patients with ovarian carcinoma develop auto-antibodies against p53 that circulate in the serum. Since many ovarian cancer patients develop ascites during their disease, we speculated that p53 antigen and/or p53 auto-antibodies may be present in this biological fluid. Ninety-six ascites fluids from women with primary ovarian cancer and one from a patient with cancer of the breast, which metastasised to the ovaries, were analysed for p53 auto-antibodies. Seventeen ascites fluids (18%) contained auto-antibodies. For 30 of these patients, serum was also available. Auto-antibodies were present in both serum and ascites in 6/30 patients; 22 patients were negative in both ascites and serum; and 2 patients had auto-antibodies only in their serum. All 97 ascites fluids were also analysed for p53 antigen and 7 (7%) were positive. None of the 17 p53 auto-antibodies-positive ascites fluids were positive for p53 antigen suggesting that p53 auto-antibodies may interfere with p53 antigen detection by capturing the antigen. In total, 24 patients (25%) had either p53 auto-antibodies or p53 protein in their ascites fluid. These data demonstrate that p53 auto-antibodies and/or p53 protein are present in ascites and may have some value for tumour diagnosis, prognosis or monitoring.