Lysophospholipase activity in brain subcellular fractions was measured by the release of myristic acid from 1-myristoylglycerophosphocholine or through the formation of [32P]glycerophosphocholine from [32P]lysophosphatidylcholine. Although the lysophospholipase activity was highest in microsomes, considerable enzyme activity was also found in other subcellular membrane fractions. The pH optimum for the microsomal enzyme was around 7, whereas the synaptosomes and non-synaptic plasma membranes exhibited a pH maximum around 8. Although the enzyme did not require divalent cations for activity, divalent cations (1 mM) such as Hg2+, Cu2+, and Zn2+ inhibited potently the enzyme activity. Enzyme activity was also partially inhibited by both saturated and polyunsaturated fatty acids (25-200 microM), and the inhibition seemed to be greater in the membrane than in the cytosolic fractions. Ionic detergents such as deoxycholate and taurocholate inhibited the lysophospholipase. On the other hand, the effect of Triton X-100 was biphasic, i.e., stimulation at concentrations below 100 micrograms/mg protein and inhibition at higher concentrations. Addition of cholesterol (50-250 micrograms/ml), but not cholesteryl esters, also potently inhibited enzyme activity. The presence of active lysophospholipase(s) in brain is probably an important mechanism for preventing unnecessary accumulation of lysophospholipids which may exert a deleterious effect on the membranes because of their detergent properties.