The cytologic evaluation of poorly differentiated tumors frequently poses a diagnostic dilemma as to the tissue of origin. To assess the diagnostic utility of monoclonal antibodies (MAbs) in these situations, we applied a panel of three highly purified MAbs specific for tumor-associated ganglioside epitopes to a diverse series of cytologic specimens. The panel was composed of DMAb-3, reactive with the epitope GalNAc beta 1-4 (NeuAc alpha 2-3)Gal- of GM2; DMAb-7, reactive with the epitope (NeuAc alpha 2-8NeuAc alpha 2-3)Gal beta 1-4(Glc or GlcNAc)- of GD3 and 3'8'-LD1; and DMAb-20, reactive with the epitope GalNAc beta 1-4(NeuAc alpha 2-8NeuAc alpha 2-3)Gal- of GD2. The cytologic material consisted of air-dried Cytospin preparations prepared predominantly from fine needle aspirates and stained with the ABC immunohistochemical method. Positive reactivity was recognized when greater than 5% of tumor cells stained with the antibody; lesser reactivity was called negative. DMAb-3 stained 9/14 (64%) glial tumors, 4/13 (31%) nonglial central nervous system tumors, 1/21 (5%) melanomas, 7/38 (18%) non-small cell carcinomas (NSCC), 1/15 (7%) small cell carcinomas (SCC), 0/9 (0%) lymphomas/leukemias, 2/10 (20%) sarcomas, 1/7 (14%) miscellaneous tumors and 2/2 (100%) reactive fluids. DMAb-7 recognized 14/14 (100%) glial tumors, 9/13 (69%) non-glial central nervous system tumors, 19/22 (86%) melanomas, 19/43 (44%) NSCC, 5/15 (33%) SCC, 2/9 (22%) lymphomas/leukemias, 6/10 (60%) sarcomas, 1/7 (14%) miscellaneous tumors and 4/4 (100%) reactive fluids. DMAb-20 stained 6/14 (43%) glial tumors, 2/13 (15%) nonglial central nervous system tumors, 1/21 (5%) melanomas, 4/38 (10%) NSCC, 0/15 (0%) SCC, 0/9 (0%) lymphomas/leukemias, 1/10 (10%) sarcomas, 1/7 (14%) miscellaneous tumors and 1/3 (33%) reactive fluids. The GD3-reactive DMAb-7 recognized a large portion of many tumor types and thus is not diagnostically useful alone. DMAb-3 and DMAb-20 were more selective and showed the strongest reactivity for glial tumors and minimal reactivity for melanomas, small cell carcinomas, and lymphomas or leukemias. DMAb-3 and DMAb-20 may be useful as components of a larger panel of MAbs in distinguishing between poorly differentiated tumors in samples derived from the central nervous system.