The efficacy of a new tissue-culture technique for the diagnosis of malignant pleural effusions was contrasted with conventional cytologic methods. The new technique is based on the ability of the tissue-culture medium to transform mesothelial cells into fibroblasts while not sustaining lymphoreticular cells. Against a background of fibroblasts, islets of adenocarcinoma are easily identified. Of 20 patients with pleural effusions and solid tumors, the tissue-culture technique correctly diagnosed eight (40 percent), compated to seven (35 percent) for conventional cytologic methods. Of 27 pleural effusions in patients without malignant neoplasms, there was only one probable false-positive.