ATP conferred protection of silver nanoparticles (AgNPs) against salt induced aggregation has been used as a simple, sensitive and cost effective tool for detection of acid phosphatase (ACP) activity. In the absence of the enzymes, unreacted ATP protects AgNPs from salt-induced aggregation. Dephosphorylation of ATP in the presence of the enzyme reduces the protecting action of ATP and thus there is aggregation of AgNPs. Developed sensing tool for the detection of ACP were found to be suitable in the range of 0.6 to 7 units per ml.