Existing variants of the oncogene v-src differ in their transforming potential as well as in the range of their hosts. We compared the protein kinase activities of two Prague C v-Src variants (PRC and H19), reported to be of low oncogenic potential (Plachý et al., 1995), with the highly oncogenic Schmidt-Ruppin A v-Src (SRA). We employed in vitro kinase assays of affinity-purified proteins expressed in rabbit reticulocyte lysate and in S. cerevisiae. In both systems used, the specific kinase activity of the Prague C v-Src kinases amounted to only ca 20% of the activity of SRA. This positions the PRC Src close to activated c-Src, despite the lack of the regulatory C-terminal tail in PRC. We constructed chimeras between PRC and SRA v-Src and tested them for specific kinase activity in S. cerevisiae. Remarkably, the regulatory N-terminal part of PRC, when fused to the SRA-derived kinase domain, lowered the chimeras' PK activity to ca 20%, suggesting that it is the regulatory part of PRC that is responsible for its low phosphotransferase activity.