By Western blot technique, 519 samples of human sera were tested for the presence of antibodies to the human papillomavirus (HPV) type 16 proteins E4 and E7 that had been expressed in Escherichia coli as fusion proteins. Sera were obtained from patients attending the University hospitals for reasons unrelated to HPV infections (controls), from patients with HPV-associated lesions, as well as from patients suffering from cervical cancer. Within the control population, 18.1% of them had antibodies that reacted with the E4 protein, and 3.9% of them had antibodies that reacted with the E7 protein. No sex-specific difference in the antibody prevalence was observed. The highest proportion of anti-E4 antibody-positive individuals (40.7%) was observed in the age group between 11 and 20 years. The frequency of anti-E4-positive sera was threefold higher in patients with HPV-associated genital lesions than that in age-matched controls. Antibodies against the HPV16 E7 protein were found 14 times more frequently in patients with cervical cancer, compared with age- and sex-matched controls (P less than .00001). From these data, we concluded that anti-E4 antibodies may be correlated with virus replication and that anti-E7 antibodies may represent a marker for cervical cancer development.