A monoclonal antibody reacting with the nucleus antigen was produced by hybridoma techniques. Distribution of the antigen during the cell cycle of amoebae was followed by immunofluorescence microscopy. During interphase, the antigen was localized on the nucleus membrane. But it was dispersed all over the cytoplasm during mitosis and cytokinesis. The molecular weight of the antigen was 210KD and 190KD as determined by SDS PAGE and transfer blotting of the purified nuclei. The antigen was not soluble in 2% Triton X-100, but was released from the nucleus by incubation with 0.05M sodium carbonate. Thus the antigen was confirmed as a peripheral protein of the nucleus membrane. The isoelectric point of this antigen was 7.64 as determined by 2D PAGE and transfer blotting. Considering the peripheral association with the nucleus membrane and the dispersed distribution during mitosis, the antigen could be a lamin like protein on one hand, but it may have some other role on the other as the molecular weight and the pI of this protein is different from the characterized lamin in other cell system.