Protein kinase CK1 (casein kinase I) is involved in the regulation of various functions including apoptosis. The specific CK1 inhibitor D4476 may either inhibit or foster apoptosis. Similar to apoptosis of nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include isotonic cell shrinkage by Cl^- removal (replacement with the impermeant organic anion glutamante) or oxidative stress (exposure to the oxidant tert-butylhydro-peroxide [TBOOH]). Western blotting was utilized to explore whether erythrocytes express the protein kinase CK1, and FACS analysis to determine forward scatter (reflecting cell volume), annexin V binding (reflecting phosphatidylserine exposure), and Fluo3 fluorescence (reflecting cytosolic Ca²⁺ activity). As a result, both, human and murine erythrocytes express the CK1 protein. Replacement of Cl^- with gluconate for 48 hours and a 30 min exposure to 0.3 mM TBOOH decreased forward scatter, increased annexin V binding and increased Fluo3 fluorescence. Addition of D4476 (2 mM) did not significantly modify forward scatter, annexin V binding or Fluo3 fluorescence in the presence of Cl^- and absence of TBOOH, but significantly blunted the increase of annexin V binding and Fluo3 fluorescence following Cl^- removal and the increase of annexin V binding following TBOOH treatment. In conclusion, CK1 is expressed in erythrocytes and participates in the regulation of eryptosis.