Serial DNA-polymerase-alpha and -beta enzyme level measurements were made during the growth of the EMT6 solid tumor. The determinations were correlated with DNA-synthesis in the tumor as measured by 3H-TdR labeling. The DNA-alpha polymerase levels were initially found to be low during early exponential growth phase, but to rise midgrowth to peak and to remain elevated thereafter until an advanced growth stage was reached. DNA-beta polymerase did not change during this entire observation period. DNA synthesis as measured by 3H-TdR uptake was greatest early in tumor growth and fell in midexponential growth phase. This pattern indicates that these two processes are not tightly coupled in the process of localized tumor growth.