A novel series of substituted (thieno [2, 3-d] pyrimidin-4-ylthio) carboxylic acids has been synthesized and tested in vitro towards human protein kinase CK2. It was revealed that the most active compounds inhibiting CK2 are 3-{[5-(4-methylphenyl) thieno [2, 3-d] pyrimidin-4- yl] thio} propanoic acid and 3-{[5-(4-ethoxyphenyl) thieno [2, 3-d] pyrimidin-4-yl] thio} propanoic acid (IC50 values are 0.1 μM and 0.125 μM, respectively). Structure–activity ...