Alterations for acid and alkaline phosphatase levels and their pattern of splenic and lymph node activity in normal and virus-induced lymphoblastic leukemia were studied. Enzyme levels were examined by using both cytochemical and biochemical procedures. The GC leukemia virus, a ribonucleic acid murine virus antigenically related to the Rauscher-Moloney viruses, was used to stimulate acid and alkaline phosphatase by producing lymphomaceous disease in Ha/ICR mice. With the Burstone and Gomori cytochemical procedures, both enzymes were found in higher than normal levels in lymphomaceous spleen and lymph nodes. Confirmation of the cytochemical studies was obtained by enzyme assay of cell-free homogenates in each case with the exception of spleen acid phosphatase. The discrepancy between the cytochemical tests which showed significant elevation of spleen acid phosphatase and the enzyme assays which failed to reveal such elevation could be due to a labile acid phosphatase isozyme which is lost on cellular disruption during homogenate preparation. A significant spleen alkaline phosphatase specific activity elevation above normal was found with a 50% incidence only when leukemic spleen wet weight increased nearly threefold its normal value. This result suggests that alkaline phosphatase elevation is a secondary event occuring after the onset of disease and is not a fundamental metabolic alteration concerned with the onset of murine lymphoblastic leukemia.