Gel filtration of soluble supernatant fraction obtained from livers of rats 10 min after an injection of the haem precursor 5-amino[3H]laevulinic acid shows the presence of a major radioactive fraction which upon gel filtration is similar in elution volume to ligandin. 20 min after administration of the precursor four previously minor components also come into prominence. This pattern is a characteristic of in vivo binding since a different elution pattern is obtained if soluble supernatant fraction from rat liver is labelled in vitro by incubation either with [3H]haem-labelled mitochondria, [3H]haem-labelled microsomes or with [3H]haemin. These results are discussed with particular reference to ligandin.