Cellular immune responses of patients with histologically confirmed lung carcinoma were assessed in vivo using cutaneous response and in vitro with a microlymphocyte blastogenic transformation (LBT) assay. In addition, correlation of the cutaneous response with the migration inhibitory factor (MIF) assay and LBT response was examined. The results indicated that cutaneous responses seen in patients with cancer of the lung were consistently lower than similar responses in normal controls (p < 0.001). Similarily, the percentage of positive cutaneous responses seen with patients included in this study was lower than the frequencies reported by others. Stimulation of cells from lung cancer patients by PHA-M was also depressed when compared to similar lymphocytic responses in normal volunteers (p < 0.001). The correlation between cutaneous response to tuberculin and the in vitro assays was high. The few instances of disparity demonstrate the need to utilize more than one assay in evaluating cellular immune functions. These data would support the work of ther that indicate a depression of cellular immunity in advanced malignancy.