In an attempt to elucidate the cellular mechanisms, the contribution of the major types of blood leucocytes to the LAI reaction was studied with breast carcinoma and control tumor extracts in patients with breast cancer and other gynecologic tumors. Lymphocytes from breast cancer patients enriched in T-cells showed, compared to monocytes, non-T-cells (B-cells) and granulocytes, a reduced adherence in the presence of extracts from breast carcinoma but not from control tumor tissues. (T)Lymphocytes from control tumor patients failed to respond to the same breast carcinoma extracts in a similar manner. The LAI effector lymphocytes from breast cancer patients showed dose-dependent blastogenic responses to T-cell mitogens. In the two-step procedure called the indirect LAI test, initial attempts failed to detect an immunological mediator released by the lymphocytes from breast cancer patients with extracts of the same tumor tissue type, as the preparation of the indicator cells (lymphocytes from control donors) proved to be critical. However, when these indicator cells were conditioned with indomethacin (0.1–10 μg/ml) or were washed in medium prior to the incubation with the active supernatants of LAI-positive lymphocyte cultures, the detection of specific adherence inhibition activity became highly reproducible. The results suggest that the LAI test measured cell-mediated immunity to tumor-type-specific antigens. Indomethacin-sensitive inhibitors might account for some previous failures to detect a soluble adherence inhibition factor (LAIF) as part of a specific immune response.