The use of membrane filters to concentrate cytology specimens was first described by Seal (Cancer 1956; 9:866-868). We report on a technique in which a portion of Papanicolaou-stained Millipore filter (Millipore Corp, Bedford, MA) preparation can be converted into a paraffin block for hematoxylin-eosin (H&E) preparations and immunocytochemical analysis (ICC). Seven cases with moderate cellularity and no cell block preparation were retrieved from our cytopathology files. The specimens included: 4 pleural effusions with metastatic adenocarcinoma, and 3 FNA specimens (1 metastatic melanoma, 1 metastatic adenocarcinoma, and 1 thyroid/papillary carcinoma). The filter was removed from the slide, cut in half, and subjected to paraffin embedding in the usual fashion (postfixed in 10% neutral-buffered formalin). Four-micron-thick sections were cut onto Probe-On (Fisher Scientific, Pittsburgh, Pa) slides. ICC was performed using the avidin-biotin-peroxidase complex technique and capillary gap technology. Antibodies included CAM5.2, AE1/AE3, CEA, CD15, LCA, PanCK, S100, HMB45, and thyroglobulin. All cases showed excellent preservation of cellular morphology on H&E. ICC performed on Millipore filter cell block preparations showed specific antibody staining patterns with preservation of cellular details. All antibodies showed their specific staining patterns with clean background and lack of nonspecific staining. This technique has the following advantages: 1) offers an alternative to cell blocks in moderately cellular specimens; 2) clean background; 3) preservation of cytology specimens for future studies.