Hemeproteins, like cytochrome c (12,500 M.W.; Karnovsky and Rice, 1969) and myoglobin (17,816 M.W.; Anderson, 1972; Simionescu et al., 1973) are advantageous over the true peroxidases with larger molecular weights (e.g. horseradish peroxidase, ca. 40,000 M.W) as ultrastructural probes in that they do not elicit vascular leakage in the inflammatory response (Cotran and Karnovsky, 1967) and are relatively nontoxic immunologically inert substances. The main disadvantage in using cytochrome c and myoglobin is that they have weak peroxidatic activity compared to the true peroxidases (Nakamura et al., 1960; Keilin, 1961; Kurozimi et al., 1961). These hemeproteins, however, offer the following advantages: 1) they retain sufficient peroxidatic activity after aldehyde-fixation to oxidize 3,3'-diaminobenzidine (DAB), 2) they may be localized by virtue of an insoluble reaction product (osmium black) deposited at the site of hemeprotein immobilization by fixation, and 3) they represent low molecular weight probes. This brief report emphasizes the advantages of myoglobin in the study of glomerular permeability, transport by endocytosis in proximal tubules and translocation of protein in the lower segments of the nephron.