Migration stimulating activity in serum of breast cancer patients. An assay to measure the ability to stimulate migration of fibroblasts into collagen gel was carried out on serum from treated and untreated breast cancer patients and from healthy controls. Migration stimulating activity was found in the serum of 10 (83%) of 12 untreated breast cancer patients immediately before surgical resection of the primary tumour and in 9 (75%) of them 4 days after resection; in 13 (93%) of 14 patients 1-13 years after tumour resection who had received adjuvant treatment; and in 2 (10%) of 20 healthy women matched for age. The migration stimulating activity in cancer patients' serum was indistinguishable from the migration stimulating factor produced in vitro by fetal and cancer patient skin fibroblasts in its behaviour in various biochemical fractionation procedures. The presence of this activity in the serum of treated breast cancer patients clearly distinguishes it from other oncofetal proteins, which all seem to be produced by tumours. 