ESR investigations of lyophilized blood of patients with acute lymphatic leukemia exhibit an increase in spin concentration and an additional peak not present in control samples. This peak disappears almost completely concomittantly with a reduction in spin concentration after treating the patients with prednisolone, vincristine, and doxorubicin or after addition of 5mM of CuCl2 to blood samples. The results show that the ESR spectra of lyophilized blood samples can be utilized to follow the effect of therapy. Moreover, they suggest that the leukemic blood possesses a high concentration of antioxidants.