C-phycocyanin (C-PC), a water-soluble protein pigment, isolated from Spirulina platensis, is of great importance because of its various medical and pharmacological properties. In the present study, we first investigated the effect of highly purified C-PC on growth and proliferation of HeLa cells in vitro. The results indicated that there was a significant decrease in the number of cells that survived for HeLa cells treated with C-PC compared with control cells untreated with C-PC. Further electron-microscopic studies revealed that C-PC could induce characteristic apoptotic features, including cell shrinkage, membrane blebbing, microvilli loss, chromatin margination and condensation into dense granules or blocks. Agarose electrophoresis of genomic DNA of HeLa cells treated with C-PC showed fragmentation pattern (DNA ladder of oligomers of 180-200 bp) typical for apoptotic cells. Flow-cytometric analysis of HeLa cells treated with different concentrations of C-PC demonstrated an increasing percentage of cells in sub-G0/G1 phase. In addition, we found that C-PC could promote the expression of Fas and ICAM-1 (intercellular cell-adhesion molecule 1) protein, while it held back the Bcl-2 (B-cell lymphocytic-leukaemia proto-oncogene 2) protein expression. This suggested that C-PC could induce the activation of pro-apoptotic gene and downregulation of anti-apoptotic gene expression and then facilitate the transduction of tumoural apoptosis signals that resulted in the apoptosis of HeLa cells in vitro. Caspases 2, 3, 4, 6, 8, 9, and 10 were activated in C-PC-treated HeLa cells, which suggested that C-PC-induced apoptosis was caspase-dependent. C-PC treatment of HeLa cells also resulted in release of cytochrome c from the mitochondria into the cytosol that was related to apoptosis of C-PC-treated HeLa cells.