The in vivo distribution of 125I and fluorescein labelled C. parvum, after local and systemic injection mice has been studied. After i.v. injection large amounts were found in the liver, spleen and lungs, with less in the lymph nodes. The distribution after s. c. injection was more restricted; the bulk of inoculum was retained at the injection site and draining lymph node, and only small amounts were found in the liver and spleen. C. parvum was found in bone marrow, but not peritoneal cells after i. v. injection. It was, however, possible to recruit C. parvum-containing cells into the peritoneal cavity by i. p. injection of thioglycollate, and to a lesser extent, i. p. injection of syngeneic tumour cells.