Human T-cell lymphotropic virus type 1 (HTLV-1) is known to be transmitted vertically through breastfeeding and horizontally by blood transfusion and sexual contact. Our intervention study has suggested the presence of additional alternative maternal transmission pathways. To explore the possibility of transmission through saliva, we used PCR to quantify the HTLV-1 provirus in saliva samples from 18 carrier mothers and 10 patients with HTLV-1-associated myelopathy/tropical spastic paraparesis. The provirus was detected in 60 and 90%, respectively, of the samples, with estimated copy numbers in the range of 10 to 10(4)/ml. However, the saliva, regardless of the presence or absence of antibodies to the virus, showed a strong tendency to inhibit the cell-to-cell transmission of HTLV-1 in vitro, as examined by a syncytium inhibition assay. The natural inhibitory activity in saliva of seronegative volunteers was heat sensitive, and most of the activity was recovered by ultrafiltration in the fraction of macromolecules with a molecular weight of more than 100,000. In addition to this natural activity, saliva of HTLV-1-infected individuals contained immunoglobulin G molecules capable of neutralizing syncytium formation. These results strongly suggested that HTLV-1-infected cells in the carriers' saliva, which contains neutralizing antibodies in addition to the natural activity inhibiting cell-to-cell viral infection, barely transmit the virus. Transmission of HTLV-1 through the saliva would thus seem to be rare, if it occurs at all.