Minimal doses of ionizing radiation delivered in vitro were shown to reduce the survival of human blood lymphocytes incubated at 37[degree]C in Carrel flasks or slide chambers. With the Carrel flask method, 5 r produced a mean % effect of approximately 35% and 2 r, a 21% effect. With the slide chamber method, 2 r produced a 13% effect, which was statistically significant.