The present invention provides a method for measuring the magnetic resonance signal of an intracellular population of predetermined nuclei in a cell-containing sample by: a) applying to the sample a first radio frequency pulse, in a magnetic field, b) after a set time interval (T.sub.I), applying to the sample a second radio frequency pulse sufficient to cause a measurable signal, and c) measuring the measurable signal for the sample so produced, which method is utilized in methods of (i) determining the effectiveness of chemotherapy, (ii) detecting and characterizing tumors, and (iii) determining cell death in a cell-containing sample.