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Cytotoxic effects of 10-ns electrical pulses (EP, 50-380 kV/cm) were analyzed in cultured U937 cells (human lymphoma). Densities of live and dead cells were compared in over 500 samples at intervals from 0.5 to 48 h post exposure. EP trains of 1-20 pulses caused a minor, if any, decrease in cell survival: 24 h post exposure, the density of live cells typically dropped just 10-20% compared with unexposed parallel control. Within studied limits, this effect did not significantly depend on the EP number, voltage, or repetition rate. However, much longer EP trains could cause a sharp survival decline. The transition from plateau to profound cell killing occurred at about 150 pulses at 150 kV/cm, and at over 1000 pulses at 50 kV/cm. Artifact-free thermometry using a fiber optic microprobe established unequivocally that cell killing by extra-long EP trains was not a result of sample heating and has to be explained by other mechanisms. Testing for specific apoptotic cleavage of poly(ADP-ribose) polymerase at scattered time intervals (1-24 h) after EP exposure produced mostly negative results. Overall, 10-ns EP caused far weaker cytotoxic effect than it was reported earlier from experiments in other cell lines and mostly with longer pulses (60 and 300 ns). The survival curve shape (i.e., the shoulder followed by exponential decline) is also characteristic for other cytotoxic factors, such as low-LET ionizing radiation, thereby possibly pointing to common mechanisms or targets.