The entrance of the fluorescence dye, propidium iodide, into murine myeloma cells after poration of the cytoplasmic membrane by application of a rotating electric field for 100 ms was monitored. The dye becomes strongly fluorescent only when bound to cellular DNA. The poration field was generated between four electrodes driven by four 90°-phase-shifted square-wave signals. Field strength and frequency were about 80 kV/m and 2.5 kHz respectively. After membrane permeabilization by the rotating field pulse, the dye entered through the electropores giving rise to intense fluorescence, initially only in the vicinity of the porated membrane areas. Different types of fluorescence pattern for the cytoplasm were found after the pulse. Spherical cells showed either a ring-shaped fluorescence increase or an increase starting at one or more points. For elongated cells the increase always occurred at membrane points close to the two ends of the long axis. These experiments show the advantage of rotating fields for biotechnological applications. More gentle field conditions can be used because these fields scan the cell surface. Scanning increases the probability of poration of randomly oriented non-spherical cells or of the weakest membrane points of electrically inhomogeneous cells. © 1992.