The effectiveness of a standard ESR broadening agent, potaspotassium trioxalatochromiate (CrOx). It was found to be sium trioxalatochromiate (CrOx), for use with the spin-label temtwice as effective as K 3 Fe(CN) 6 . Although the toxicity of pone, was compared to that of diethylenetriaminepentaacetic acid CrOx is much less than that of NiCl 2 and K 3 Fe(CN) 6 , it is (DTPA) containing an ion (Gd, Cr, Mn, Fe) with a large magnetic not ideal. For instance, it can quickly immobilize up to 25% moment. Signal attenuation, line broadening, toxicity, and cell of human sperm (7). membrane permeability were compared. As a broadening agent, A number of other broadening agents have been suggested CrOx was most effective, followed by Fe-DTPA. CrOx proved or found useful in biological ESR studies. Sodium-mangamildly toxic while Gd-DTPA and Fe-DTPA were virtually nonnese-EDTA has been used to investigate osmoregulation in toxic. The human red blood cell membrane was tested for permecyanobacteria (13), MnO-EDTA has been suggested for ability to Fe-and Gd-DTPA and found to be impermeable to the measurement of mammalian sperm intracellular water both. In situations where toxicity to cells is critical, the DTPA chelates, particularly Fe-DTPA, may prove an acceptable substi-(14), and KMn-EDTA has been used in an investigation tute for CrOx.