Monoclonal antibodies were prepared to study the cytoplasmic face of latex phagolysosomes isolated from thioglycollate-elicited mouse peritoneal macrophages. Phagolysosomes obtained by sucrose flotation contained latent beta-glucuronidase activity and tightly associated cellular proteins and glycoproteins. Fluorescence-activated cell sorter analysis, scanning and transmission electron microscopy showed that the particle preparation contained greater than 98% monomers and dimers, invested with a smooth layer of membrane and minimally contaminated with cytoplasmic adhesions. Sera for immunized rats bound preferentially to isolated phagolysosomes rather than intact cells and monoclonal antibodies PL-1 and PL-4 were isolated on this basis. Indirect fluorescent, radio- and peroxidase immunobinding assays with intact and methanol-permeabilized cells confirmed that antigens PL-1 and PL-4 were exclusively intracellular and that well-washed phagolysosomes bound both antibodies. These antigens were found in a variety of cells from several species and in macrophages not fed latex. Although the PL-1 antigen could not be immunoprecipitated, intracellular staining was characteristic of intermediate filament distribution, that is, it was in the form of a fine intersecting network, which collapsed, reversibly, in a rim round the nucleus upon treatment with colcemid. The staining pattern was undetectable in cells 1 h after adherence to a substratum, but gradually appeared after 6-12 h. The PL-4 antibody has been shown elsewhere to define a Ca2+-binding protein of approximately 20 000 molecular weight, which is phosphorylated during phagocytosis. This antibody stained stress fibres and revealed a widespread punctate distribution of antigen within cells at all stages after adhesion. The nature of the association between these intracellular antigens and phagolysosomes and their possible role in phagocytosis are not known.