SummaryArtery-to-vein and artery-to-urine transit characteristics of 125I-myoglobin across the rabbit kidney were compared to those of cadmium-metallothionein (CdMT) labelled with 109Cd, and their interaction during tubular reabsorption was determined. Both proteins are reabsorbed by a saturable system, mutually inhibit each other's reabsorption, and suffer similar tubular transit delays. On the basis of these results, and of the previous observation that myoglobin inhibition of CdMT reabsorption is fully reversible, we may tentatively conclude that the two proteins compete for reabsorption by a common transport system. This system also reacts with hemoglobin, indicating that its affinity for substrates is determined by factors other than purely size of the protein molecule.