Long interspersed element-1 (LINE-1) is the only active protein-coding retrotransposon in humans. It is not expressed in somatic tissue but is aberrantly expressed in a wide variety of human cancers. ORF1p protein is the most robust indicator of LINE-1 expression; the protein accumulates in large quantities in cellular cytoplasm. Recently, monoclonal antibodies have allowed more complete characterizations of ORF1p expression and indicated potential for developing ORF1p as a clinical biomarker. Here, we describe a mouse monoclonal antibody specifi c for human LINE-1 ORF1p and its application in immunofl uorescence and immunohistochemistry of both cells and human tissues. We also describe detection of tagged LINE-1 ORF2p via immunofl uorescence. These general methods may be readily adapted to use with many other proteins and antibodies.