In order to investigate the relationship of different cytokeratins within one cell, monoclonal antibodies directed against three trophectoderm cytokeratins TROMA 1, 2 and 3 were microinjected into mouse teratocarcinoma-derived trophoblastoma cells and indirect immunofluorescence tests were used to follow the subsequent localization of their respective antigens Endo A, B and C. Microinjection of TROMA 1 or 2 resulted in the perinuclear collapse of Endo A, B and C-containing filaments. Microinjection of TROMA 3 resulted in the perinuclear collapse of filaments containing Endo A and B, whereas Endo C condensed into cytoplasmic aggregates which appear as speckles in the fluorescence microscope. The speckles were electron microscopically located using indirect gold-labeling techniques and had a dense, granulous structure. They were often found to be associated with microtubules, although colchicine treatment before microinjection did not interfere with speckle formation. These experiments demonstrate that cytokeratins can become differentially distributed within the cytoplasm after microinjection of an anti-cytokeratin monoclonal antibody. Since Endo A is a type II cytokeratin and Endo B and C are type I cytokeratins, these results suggest that different members of one cytokeratin subfamily may be associated with cytokeratin filaments which have different functions within the same cell.