We have developed 2 new quantitative methods for measuring anti-p53 antibodies in human serum. Using these methods we analyzed 1,392 sera from patients with various malignancies and 230 sera from individuals without malignancy. Highest prevalence of anti-p53 antibodies was associated with ovarian and colon cancers (15%), followed by lung (8%) and breast (5%) cancers. Prevalence in other malignancies was lower (< 4%). In hospitalized patients and apparently healthy individuals, prevalence was very low (< 2 and 1% respectively). Extremely high antibody concentrations (> 10(5) U/L) were found in 5 ovarian, 2 breast, 1 lung and 1 colon cancers. Sequential analysis of 6 positive samples has shown that the p53 antibody test may have potential for patient monitoring. The p53 antibody-positive sera from breast cancer patients were associated with tumors that were steroid hormone receptor-negative (p < 0.002). We propose that the measurement of p53 antibodies is a relatively specific serological test for cancer, which can be performed with easily automatable and quantitative methodologies and may be further exploited for patient monitoring, prognosis, diagnosis and probably screening for selected cancers.