The isolation of Cowdria ruminantium by differential and isopycnic density gradient centrifugation is reviewed with special reference to the suitability of Percoll as density gradient medium. Infected sheep brain,
Amblyomma hebraeum nymphae and various mouse organs were used as starting material. By these methods,
partially purified viable populations of the organism with distinctly different densities were obtained. The
conclusions are based upon results of analyses of density fractions by inoculation into sheep or mice, protein
determination, electron microscopy and enzyme-linked immunosorbent assay. Morphological differences were
observed in the density fractions obtained from infected brain tissue and A. hebraeum.