Selective incorporation of 13C into the methyl groups of protein side chains is described as a means for simplifying the measurement and interpretation of 13C relaxation parameters. High incorporation (> 90%) is accomplished by using pyruvate (3-13C, 99%) as the sole carbon source in the growth media for protein overexpression in E. coli. This improved labeling scheme increases the sensitivity of the relaxation experiments by approximately fivefold when compared to randomly fractionally 13C-labeled protein, allowing high-quality measurements on relatively dilute (< 1 mM) protein samples at a relatively low cost.