The modulation of a brain phosphoinositide-specific phospholipase C-alpha activity was studied using a variety of compounds of different charge. Detergents such as sodium deoxycholate and cetyltrimethylammonium bromide stimulated the phospholipase C activity when used alone but when used together the effects were not additive. Spermine was an effective inhibitor of the enzyme activity while the cationic peptide, Melittin, had no effect. The inositol phosphates produced by hydrolysis with phosphoinositide-specific phospholipase C were inhibitory while diacylglycerol and inositol did not affect the phospholipase activity. Myelin basic protein, which was previously shown to stimulate phospholipase C activity by 2.5-fold, did not interact with the anionic inositol phosphatases to any significant extent. Thus we concluded that the mechanism of stimulation was not due to relief of product inhibition. Crosslinking studies with the photoactivatable reagent, N-hydroxysuccinimidyl-4-azidosalicylic acid, showed that peptide 24-33 of myelin basic protein, which stimulated the activity almost as much as the native protein, interacted specifically with the phospholipase C. Thus the mechanism by which myelin basic protein stimulated the enzyme appeared to be through specific protein-protein interaction.