The proportion of peak I (molecular weight > 107) to peak II (molecular weight ca. 105) invertase increases in the leaves of common wheat,Triticum aestivumL. emend. Thell ssp.vulgare, as they age. Elongating leaf tissues produced at 21 °C contain much higher levels of peak I plus peak II invertase than do mature leaves. Most of this invertase is peak II material. In cold-hardened plants, old leaf tissues contain much more peak I plus peak II invertase than do young or elongating leaves. Such old leaves contain mostly peak I invertase.The ratio of peak I to peak II invertase in the leaves or roots from cold-hardened plants is higher than that from comparable tissues of non-hardened plants.Earlier data showed that the ratios of the quantities of peak I to peak II invertase are highly correlated with cold hardiness when a group of common wheats grown under the same hardening conditions are compared but not when a single variety is grown under different hardening conditions. The present data show that the ratio of peak I to peak II invertase changes as the tissues age. Consequently, comparing the ratios of peak I with peak II invertase in plants consisting of tissues that are not of comparable ages will confound the effects of tissue age and tissue cold hardiness. Good correlations between cold hardiness and the ratio of peak I to peak II invertase cannot be expected and were not found in earlier work under such conditions.