High-energy, high-charge nuclei may contribute substantially to the yearly equivalent dose in space flight from galactic cosmic radiation (GCR) at solar minimum. The largest single heavy-ion component is 56Fe. We used the mouse embryo chimera assay to test 512 MeV/u 56Fe nuclei for effects on the rate of proliferation of embryonic cells transmitted by sperm from irradiated mice. Male CD1 mice were acutely irradiated with 0.01, 0.05 or 0.1 Gy (LET, 184 keV/micron; fluence, 3.5 x 10(4)-3.3 x 10(5) nuclei/cm2; average dose rate, 0.02 Gy/min) at the Lawrence Berkeley Laboratory BEVATRON/BEVALAC Facility in Berkeley, CA. Irradiated males were bred weekly for 7 weeks to nonirradiated females and their four-cell embryos were paired with control embryos, forming aggregation chimeras. After 30-35 h of culture, chimeras were dissociated to obtain "proliferation ratios" (number of cells contributed by the embryo from the irradiated male/total number of cells in the chimera). Significant dose-dependent decreases in proliferation ratios were obtained across all three dose groups for postirradiation week 2 (P < 0.05 to P < 0.003). The 0.01- and 0.05-Gy dose groups also produced significant decreases in proliferation ratios for postirradiation week 1 (P < 0.05 to P < 0.01) and the 0.05-Gy dose group produced significant decreases in proliferation ratios for postirradiation week 6 (P < 0.05). Postirradiation weeks 1, 2 and 6 correspond to irradiation of epididymal sperm, testicular spermatids and spermatogonia, respectively. We calculate that only about 5% of sperm in the 0.1-Gy, 2.5% in the 0.05-Gy and 0.5% in the 0.01-Gy dose groups sustained direct hits from 56Fe nuclei. However, up to 47% of sperm during postirradiation weeks 1 and 2 transmitted proliferation ratios that were at or below one standard deviation from control mean proliferation ratios. Morphometry on sectioned testes showed a significant log-linear dose response for cell killing of type B spermatogonia, which are the most radiosensitive stage of spermatogenesis and which would have been tested as mature sperm during postirradiation week 6. We conclude that amplification from secondary radiation produced in the mouse and/or from diffusible chemical products arising from hit sperm and adjacent cells contributed to the high incidence of transmitted effects on proliferation of embryonic cells.