Leukocyte alkaline phosphatase (LAP) score in peripheral blood was determined in 45 new cancer patients, 30 with breast cancer and 15 with colorectal cancer, with nonmetastatic disease. The LAP scores were performed immediately after diagnosis or surgery, and later at intervals of 1-3 months, until clinical detection of metastases. The preliminary data presented here show that there may be some utility in measuring LAP score in patients with solid tumors on a serial basis to detect evidence of metastatic disease prior to its clinical recognition. In 22 out of 30 breast cancer patients and 11 out of 15 colon cancer patients, there were 'alarming signals' of metastases (defined in this study) in the data taken before the checkup in which metastases were diagnosed by other methods. We conclude that LAP scores should be introduced into routine checkup of breast and colon cancer patients and could be a helpful nonspecific additional element in detecting earlier metastatic disease during the follow-up of a patient. As an extrapolation from this study we suggest that work should be undertaken to explore the possibility that a sudden rise of LAP score in an otherwise healthy person, who has no known reason for an elevated LAP score, might be the very first measurable sign of cancer.