In this issue of the Journal, Chodak and his co-workers1 report that urine from patients with carcinomas of transitional epithelium induced greater mobilization of cultured capillary endothelium than did urine from a variety of controls. For a full appreciation of these results, a brief outline of previous observations related to this area of research is pertinent. In 1977 Albrecht-Buehler reported an ingenious method for quantitating the mobility of cultured cells. The procedure is based on the observation that cells moving on a glass substrate covered with gold particles form characteristic particle-free tracks that can be recorded and measured. Applied . . .