The S-4 experiment carried out in connection with the Gemini III manned spaceflight was repeated in connection with the Gemini XI mission. The S-4 experiment, designed to test the hypothesis that a radiobiological synergism exists between ionizing radiation and some other parameter associated with spaceflight, used both single- and multiple-break chromosome aberrations as biological end points. The Gemini XI experiment results, like those of Gemini III, failed to show any increase in multiple-break aberration yields when the cells were irradiated during flight. Unlike the Gemini III results, however, the Gemini XI results showed no significant increase in the yields of single-break aberrations induced by in-flight irradiation. It is concluded that the significant difference seen in the Gemini III experiment must have resulted from random sampling error and that the postulated synergism is not demonstrable for either class of human chromosome aberrations.

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