Using a beta-tubulin specific antibody, centrosomes were labeled in paraformaldehyde fixed human lymphocytes. Cells were kept in suspension to preserve the three-dimensional (3D) morphology as much as possible. The centrosome was generally identified as the focus of the microtubule array. Resting (G0) and phytohemagglutinin activated cells in G1 stage were taken for 3D analysis of the centrosome position, using confocal microscopy and 3D analysis software. Measurements were performed in relation to the nuclear center and the periphery of the propidium iodide stained area ("nuclear envelope"). The distribution of the distances between the centrosome and the nuclear center revealed that in most resting cells the centrosome was located at the basis of a nuclear indentation. Upon activation, however, the centrosome appeared to move out of the indentation during transition from G0 to G1 stage.