In a group of patients with a range of degree of advancement of renal failure, the red cell survival estimated as the half-life of radiochromium-labelled red blood cells (T<sub>½</sub>.<sup>51</sup>Cr) was correlated with several indices of renal failure. The T<sub>½</sub>. <sup>51</sup>Cr was not improved by a dialysis programme which was otherwise adequate. Furthermore, the correlation of a T<sub>½</sub>. <sup>51</sup>Cr to the biochemical indices of renal failure was lost which might suggest that some other factor or factors than those measured was the ultimate correlate with red cell survival. This work adds to the circumstantial evidence in favour of a guanidine or a similarly behaving molecule being the haemolyzing agent. The additional premise is extended that a methionine-rich diet will inhibit the haemolytic factor.