Quantitation of leucocyte alkaline phosphatase activity has been found to be a useful index in the diagnosis and the monitoring of activity in a variety of disease states. Low levels are found in some haematological disease conditions, but elevated values are found in acute stress states, in pyogenic infections, myocardial infarction, trauma, diabetes mellitus etc. Studies on the pattern of activity in solid neoplasms are scanty and the published results are often contradictory. Observations made on nine different groups of solid neoplasms suggest that certain malignant tumours are associated with elevated LAP activity levels. These high levels fail to return to normal values following treatment even when there are no clinical signs of residual tumour or of recurrence. The results suggest that while LAP quantitation has a useful role in the detection of certain malignant neoplasms, it is not a sensitive tool for monitoring tumour reactivity of quiescence.