We have shown previously that when confluent cultures of mammalian cells are exposed to very low fluences of alpha particles, fluences whereby only 1-3% of the cell nuclei are traversed by a particle, genetic effects, including specific gene mutations and sister chromatid exchanges, are induced in neighboring, nonirradiated ("bystander") cells (H. Nagasawa and J. B. Little, Cancer Res., 52: 6394-6396, 1992; H. Nagasawa and J. B. Little, Radiat. Res., 152: 552-557, 1999). The present experiments were designed to determine whether signaling pathways arising in the cell membrane may mediate this effect. Cells were irradiated in the presence of Filipin, an agent that disrupts lipid rafts, effectively inhibiting membrane signaling, and the induction of sister chromatid exchange and HPRT mutations by very low fluences of alpha particles (mean doses 0.17-0.5 cGy) was measured. Filipin completely suppressed the induction of both genetic effects in bystander cells. After exposure to 10 cGy, when most mutations occurred in directly irradiated cells, no suppressive effect of Filipin was observed. These results suggest that membrane signaling may play an important role in the bystander effect of radiation. On the other hand, the effects in directly irradiated cells do not appear to be mediated via the cell membrane.