Sera from 128 Mexican cervical cancer patients (age 30-80; mean 53.6) and from 47 healthy women (age 25-69; mean 49.2) were investigated using a newly developed assay for the detection of serum antibodies to the human papillomavirus (HPV) type 16 early protein E7. This test (CIPA), based upon immunoprecipitation followed by Western blot analysis, uses the complete E7 protein expressed in HeLa cells infected with recombinant vaccinia virus. To determine the sensitivity and specificity of this assay, these results were compared with previous results of the same sera tested by enzyme-linked immunosorbent assay (ELISA; using synthetic peptides derived from HPV 16 E7) and radio-immunoprecipitation (RIPA) using in vitro translated HPV 16 E7 protein. CIPA (45% positives) demonstrated a significant increase in detection rate compared to the peptide-ELISA (30% positives; P = 0.014, chi2-test) and only a slight increase compared to RIPA (38% positives; P = 0.204, chi2-test). Based on the testing of sera from patients with HPV 16 DNA positive tumors the specificity and sensitivity of the CIPA were 0.98 and 0.59, respectively.