The mobility of 13C specifically labeled branched chain end groups of iso-even fatty acids in intact, live Bacillus thuringiensis cells was studied by 13C nuclear magnetic resonance spectroscopy. This study apparently represents the first direct observation of branched chain carbon atoms in living cells. End groups were labeled using DL-[beta, delta, delta'-13C]valine as a precursor chain initiator for iso-even fatty acid synthesis after using L-[delta, delta'-14C]L-valine to determine optimal conditions for labeling of the membrane fatty acid end groups. Cell survival in the NMR was determined for various lengths of time at 28 and 39 degrees C. Subsequently, 13C-labeled vegetative cells, sporulating cells (three stages of development), and purified mature spores were analyzed by 13C NMR using corresponding unlabeled cells as controls. Spin lattice relaxation times (T1) were obtained for the enriched iso-branched region at 23.3 ppm and for the natural abundance peak for the glycerol backbone (carbons 1 and 3) of the membrane lipids at 61.7 ppm. The T1 of the glycerol carbons (0.08 s) did not change significantly with stage of development or temperature. The T1 of the iso-even enriched end group changed dramatically from vegetative cells (0.70s) to sporulating cells (0.28 s) at 28 degrees C. A decrease in the T1 was also observed at 39 degrees C from 0.91 s for vegetative cells to 0.54 s for sporulating cells. Accompanying the reduced mobility indicated by the T1 values, there was a general decline in the signal-to-noise ratios of identically acquired spectra as sporulation continued which culminated in the lack of discernible plasma membrane lipid resonances in purified mature spores. The progressive loss of signal appeared to have resulted from a continuous decline in the fraction of plasma membrane fatty acids with sufficient mobility to give signals above background.