Spectra have been obtained of 13 main-sequence-turnoff stars in the old open cluster NGC 188. Measurements of the Ca II H and K absorption and the 3883 CN and 4300 CH band strength show tight correlations with B-V, the scatter in which can be explained entirely by observational error. The data thus provide no evidence for the existence of any abundance spread within NGC 188. Comparison with spectrum-synthesis computations sets an upper limit of 0.10-0.15 dex to the dispersion of abundance in the cluster. This result is in stark contrast to that reported by Bell, Hesser, and Cannon (1983) for the globular cluster 47 Tucanae, where marked CN differences driven by nitrogen-abundance variations of 0.7 dex exist near the main-sequence turnoff - indicative once again of a basic difference in the chemical behavior of the old open clusters and the globular clusters. The chemical homogeneity of the turnoff stars in NGC 188 is also in contrast with the inhomogeneity suggested by the spread in colors of the red giants in this system. Possible explanations for the difference is discussed.