The sequential development of C. ruminantium (Kwanyanga and Kümm isolates) was followed in caprine leukocyte cultures by light microscopy, direct immunofluorescent microscopy (DFA), indirect immunoflourescent microscopy (IFA) and transmission electron microscopy (TEM). During the febrile response, one to several small cocci, large ring forms or rods were observed in neutrophils in blood smears and cytopreparations of neutrophil fractions using Diff Quik stain, Giemsa stain, DFA and TEM. One to several C. ruminantium colonies were seen in up to 35% of neutrophils maintained in vitro for 18 h to 5 days. The organisms were located in neutrophil phagosomes by TEM and were enveloped by two trilamellar unit membranes. Initially, C. ruminantium was tightly enclosed within phagosomes. At 20 h of incubation, organisms were frequently observed undergoing binary fission within enlarged phagosomal vacuoles. At later time periods, neutrophils harboured fully formed colonies (morula) containing numerous organisms. An occasional C. ruminantium-infected macrophage (Kümm isolate), and an occasional infected eosinophil (Kümm and Kwanyanga isolate) were found.