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Many students of the problem have been puzzled and fascinated by the possible pathological and clinical significance of certain cells found to be circulating in the peripheral blood of patients known to harbour malignant diseases of infinite variety. Theoretical and practical aspects of this matter have been discussed previously.1Almost every facet of these studies remains to date open to doubt and conjecture. These doubts revolve not only around the frequency2-7 with which such cells can be recovered but are expressed also concerning their pathological identification2,4,8,9 and consequently their prognostic implications.The variation in frequency can logically be related to the fortuitous timing of the samplings and their small volume, and can also be explained partially on the more factual bases of differences in the technique of recovery on cytological examination and the nature and extent of the specific diseases under study. Not only is the volume