Neutrophil alkaline phosphatase (NAP) activity was estimated in 50 healthy humans and 89 patients with leukaemia; 41 cases of acute myeloid leukaemia (AML), 22 cases of chronic myeloid leukaemia (CML), and 26 cases of lymphoid leukaemia (LL). The groups proved to be separate entities (p less than 0.000 25) and the differences between the groups were statistically significant (p less than 0.001) except for the difference between AML and LL. The 95% confidence limits for normal NAP scores were 15.0-132.6. Decreased scores were demonstrated in 73% of CML, 7% of AML but never in LL patients. Increased scores were found in 37% of AML, 31% of LL but never in CML patients. Evaluation of the distribution of the single cell NAP activity (negative, weak positive, strong positive) showed decreased activity in 77% of CML, 15% of AML but never in LL patients. Increased activity was demonstrated in 63% of AML, 54% of LL and 9% of CML patients. The evaluation of single cell activity is a time-saving method, which furthermore proved superior to the scoring method in discriminating between the types of leukaemia investigated.