Amalgamation of two endocytic probe techniques: fluoresceinated cationized ferritin can show up, sequentially, selected structures, first in living cells and then by electron microscopy.

Fluorescein isothiocyanate-cationized ferritin (FITC-CF) has hitherto been used mainly to identify structures in living cells by light microscopy, by virtue of its fluorescent properties. We show here that this conjugate can be used, after immediate fixation of the same cell sample and preparation of thin sections, to recognise the same structures, by virtue of the ferritin's electron opacity. The conjugate should thus have a new use as a single-application, dual-purpose probe, e.g. in endocytic studies. The procedure may have advantages over similar dual-purpose probes in not requiring staining or special treatment.