Interaction of cationized ferritin with the anionic groups on the cell surface of Naegleria was studied using transmission electron microscopy in conjunction with fluorescence microscopy. Most of the experiments involved the use of fluorescein-labelled cationized ferritin (FITC-CF) incubated with living amoebae. Initially the FITC-CF was located over the posterior two-thirds of the amoebae but the label was rapidly redistributed to form a cap at the posterior end in the region of the uroid; frequently this cap was shed. Pinosomes containing FITC-CF were clearly visible within the amoebae. Amoebae prefixed in glutaraldehyde were uniformly stained and did not show redistribution of the label. Exposure of live amoebae, previously incubated with cationized ferritin and allowed to cap, to fresh FITC-CF failed to produce fluorescent staining of the general cell surface, i.e. depletion of binding sites had occurred. The binding of the FITC-CF was not affected by pretreatment of the amoebae with neuraminidase or pronase. The possible nature of the anionic sites on the membrane is discussed.