HeLa cell nuclei were isolated and the nuclear matrix specimens were prepared. After labelled with an anti-actin antibody and FITC-conjugated secondary antibody, both the nuclei and nuclear matrix specimens were observed to emanate specific yellow-green fluorescence. The fluorescent signals in the nuclear matrix were much stronger than that in the nuclei, and the signals in former were widespread throughout the whole structure while that in the latter were mainly defined to their peripheral regions. A 43 kD band was revealed in nuclei and nuclear matrix specimens by SDS-PAGE, and was then proved to be actin by Western blot, confirming the immuno-fluorescence observations. When stained with TRICT-conjugated phalloidin, both the nuclei and nuclear matrix specimens were found to give off specific, red fluorescent signals which represent the location of F-actin (filamentous actin), and the pattern of TRITC signals distribution in the nuclei and nuclear matrix showed similarity with that of FITC signals. The existence and significance of actin and F-actin in the nuclei and nuclear matrix were discussed.