It is widely accepted that fatty acid amide hydrolase (FAAH) plays a central role in the hydrolysis of anandamide. However, we found a second N-acylethanolamine hydrolase in animal tissues which hydrolyzed anandamide at acidic pH. This "acid amidase" was first detected with the particulate fraction of human megakaryoblastic CMK cells, and was solubilized by freezing and thawing without detergent. The enzyme was distinguishable from FAAH in terms of (1) the optimal activity at pH 5, (2) stimulation by dithiothreitol, (3) low sensitivity to two FAAH inhibitors (methyl arachidonyl fluorophosphonate and phenylmethylsulfonyl fluoride), and (4) high content in lung, spleen and macrophages of rat. The acid amidase purified from rat lung was the most active with N-palmitoylethanolamine among various long-chain N-acylethanolamines. To develop specific inhibitors for this enzyme, we screened various analogues of N-palmitoylethanolamine. Among the tested compounds, N-cyclohexanecarbonylpentadecylamine was the most potent inhibitor which does-dependently inhibited the enzyme with an IC(50) value of 4.5 microM without inhibiting FAAH at concentrations up to 100 microM. The inhibitor was a useful tool to distinguish the acid amidase from FAAH with rat basophilic leukemia (RBL-1) cells that express both the enzymes.