1. DNA polymerase alpha (pol alpha) isolated from Simian virus 40 (SV40)-transformed cells showed more than 3-fold higher specific activity than pol alpha from normal cells. The enzymes from untransformed and transformed cells also differed in molecular size, thermolability, sensitivity to inhibitors and specificity of template-primer utilization. 2. Western analysis using anti-Tag to probe both a crude cell homogenate and partially purified pol alpha from SV40 transformed cells showed multiple immunoreactive bands with different molecular sizes. 3. While alpha polymerases from both normal and transformed cells exhibited tightly associated primase activity, they showed different DNA binding affinities. 4. These data suggest that T antigen binding to pol alpha alters the initiation of DNA replication and/or the function of pol alpha in SV40-transformed cells, and that pol alpha from SV40-transformed human fibroblasts have different catalytic subunit characteristics than pol alpha from untransformed cells.