By labelling cells with fluorescein isothiocyanate (FITC) and other fluorescent dyes and examining frozen sections of several organs of the recipients with an ultraviolet microscope, it has been demonstrated that viable tumour cells arrive in all organs examined within 15 min of inoculation either intravenously or arterially and are still detectable in various organs 30 days later. Although the method is not quantitatively accurate, its advantages are that the cells can be directly visualised and it can be confirmed that the label is attached to whole viable cells and not to cellular fragments. The findings effectively dispose of the possibility that the consistent absence of deposits in certain sites in animals inoculated with cells from spontaneous murine mammary tumours is due to failure of the cells to reach them. It has also been confirmed that FITC-labelled cells are still capable of forming deposits and occasional fluorescent cells are detected in these secondary neoplasms.