[3H]Mianserin binding sites were characterized in membranes of dog frontal cortex and solubilized with digitonin. The solubilized [3H]mianserin binding sites retained the binding characteristics of the membrane preparation. The dissociation constant (KD) for the soluble material was 2.2 nM which compared closely to that of the membrane-bound site (1.1 nM). Drug binding profiles indicated that the solubilized sites were both serotonergic and histaminergic in nature. Drugs active both H1- and S2-receptors potently displaced [3H]mianserin with Hill slopes close to 1. More selective serotonergic drugs such as LSD and spiperone competed for [3H]mianserin binding with shallow displacement curves. Of the neurotransmitters tested, serotonin was the most potent for both preparations. Because all the components of [3H]mianserin binding co-solubilize, the soluble material can be used to isolate individual sites.