ZAP 70-expression of T lymphocytes exposed to different gravity conditions measured by FACS-analysis. Non-activated and Concanavalin A (ConA)/CD28-activated CD4+ T lymphocytes were exposed to altered gravity. Samples were stained against ZAP 70 and analyzed by FACS; data are expressed as relative fluorescence intensity (RFI). A: Non-activated (−) T lymphocytes exposed to microgravity (μg) and to an on-board reference centrifuge (1g) showed a significant reduction of ZAP 70 staining compared to the cells of the hardware ground control (H/W). ZAP 70 staining was not significantly different between 1g and μg samples. B: ZAP 70 staining of ConA/CD28 activated (+) T lymphocytes was neither significantly different between 1g and μg samples, nor between baseline (BL) samples and 1g or μg samples. The ZAP 70 staining was significantly decreased between culture controls (CC) and H/W samples, and between H/W samples and baseline (BL). Medians and interquartile ranges are shown (* p < 0.1, ** p < 0.05, ns = not significantly different, two-tailed Mann–Whitney-U-Test).