Proteomic studies of cells via surface-enhanced laser desorption/ionization spectrometry (SELDI) analysis have enabled rapid, reproducible protein profiling directly from crude samples. We applied this technique to archival cytology material to determine whether distinct, reproducible protein fingerprints could be identifiedfor potential diagnostic purposes in blinded specimens. Rapid Romanowsky-stained cytocentrifuged specimens from fine-needle aspirates of metastatic malignant melanoma (with both known cutaneous primary and unknown primary sites), clear cell sarcoma, and renal cell carcinoma and reactive effusions were examined using the SELDI technology. A unique characteristic fingerprint was identified for each disease entity. Fifteen "blinded" unknown samples then were analyzed. When the protein profilefingerprints were plotted against the known fingerprints for the aforementioned diagnoses, the appropriate match or diagnosis was obtained in 13 (87%) of 15 cases. These preliminary findings suggest a substantial potential for SELDI applications to specific pathologic diagnoses.