Since short polymer chains have a higher mobility than long molecules, conventional expectations are that the growth rate, G, of polymer crystals should decrease as the concentration of large chains increases in a binary blend. Here we present results on G as the blend concentration, phi, is varied from short chains of poly(ethylene oxide) (PEO), which are well above the entanglement molecular weight, to long PEO chains. Contrary to the simple mobility argument, G(phi) is nonmonotonic-clear evidence that another mechanism can dominate. We propose a tentative model based on the simple idea that chain ends retard the crystal growth. Thus, increasing the chain end concentration with the addition of short chain molecules can reduce the crystal growth rate.