The phosphoromonothioate oligonucleotide HPV (human papilloma virus) sequence (monothioate HPV) 5'-TTG,CTT,CCA,TCT,TCC,TCG,TC-3' was photocoupled via three different sites (the 5'-end, the 3'-end and the midpoint) to PPD (purified protein derivative) and OA (ovalbumin), and the three types of conjugates (5'-HPV/carrier, 3'-HPV/carrier and midpoint-HPV/carrier) were used for the immunization of mice. Furthermore, a group of mice were immunized with the HPV sequence alone. No detectable antibody response against the monothioate HPV oligonucleotide was seen in mice receiving only the unconjugated monothioate HPV sequence. The OA-coupled monothioate HPV sequence also failed to elicit a detectable antibody response against the monothioate HPV oligonucleotide. However the PPD-conjugated monothioate HPV sequences induced a significant anti-monothioate HPV antibody response in BCG (bacille Calmette Guérin)-primed mice, a result that must be ascribed to the effect of using PPD as a carrier in BCG-primed mice. The antisera from all groups were tested on plates coated with the corresponding OA conjugates. By far the strongest response was obtained in mice receiving the HPV sequence coupled at the midpoint position. Further, all three groups of antisera obtained by immunizing with the different PPD conjugates were tested on microtiter plates coated with one of the three different OA conjugates. The antisera differed in their response depending on which OA conjugate was used for coating of the plate. Again, the midpoint-HPV/PPD antiserum showed the highest response, and this conjugate apparently represents the most efficient immunogen. Results from inhibition experiments with various relevant analogs of the monothiate HPV sequence showed that the three antiserum pools contained antibodies predominantly directed against the conformation of the monothioate backbone structure, but that at least a subpopulation of the antibodies recognized structures, which depended on the specific HPV base sequence.