The content of biologically active hemoglobin fragment neokyotorphin (TSKYR) as well as that of neokyotorphin fragment (1-4) (TSKY) were determined in extracts of lung, heart, and brain tissue of rats. The content of both peptides as well as the neokyotorphin/neokyotorphin(1-4) ratio differed significantly from each other in these tissues. The respective parameters deviate considerably from those of erythocytes where these peptides are originally formed. Comparative analysis of cytolytic activity of peptides was performed at human erythroid leukaemia (K562) and murine transformed fibroblast (L929) cell lines. TSKY showed reliable cytolytic activity in both cell lines, while neokytorphin was not cytotoxic. The data obtained lead to speculation that endogenous hemoglobin fragments might participate in regulation of tumor growth in vivo.