Abstract: A method for diagnosing or differentially diagnosing a cancer characterized by the presence of cancer cells in the pleural fluid of a mammalian subject, the method comprising contacting a sample of pleural fluid of the subject with colloidal magnetic particles coupled to a ligand which binds to a determinant on a cancer cell, but does not bind above a baseline threshold to other cellular and non-cellular components in pleural fluid; subjecting the pleural fluid-magnetic particle mixture to a magnetic field to produce a cell fraction enriched in ligand coupled-magnetic particle-bound cancer cells, if present in the pleural fluid; and analyzing the enriched fraction for the number of cancer cells in the pleural fluid. In certain aspects, this method involves preparing the pleural fluids for the above-noted method steps by, e.g., dilution of unprocessed pleural fluid. In certain aspect, the pleural fluid is subjected to the diagnostic method within 24 hours of withdrawal from the subject.