Frequently peripheral blood lymphocytes (PBL) do not reflect the tumor host relationship and cell mediated immunity in the PBL does not often correlate with prognosis. The tumor infiltrating lymphocytes (TIL) interact most closely with the tumor cells and are likely to more accurately reflect tumor host interactions. These studies indicate that TIL from pulmonary tumors are similar to PBL so far as their cell surface markers are concerned. The percentage of T-cells, B-cells, helper cells, suppressor cells, and NK cells are similar in the two compartments. However, the TIL are markedly suppressed in their functional capacity as measured by their proliferative and cytotoxic activity. In addition, natural killer (NK) cell activity is markedly diminished in TIL as opposed to the PBL. In addition, the direct injection of BCG into these tumors reverses this phenomenon by significantly increasing T-cell and NK cell functional activity. Thus, the microenvironment of the tumor profoundly affects the immunologic relationship between the tumor and the host.