The binding constants of 15 phenothiazine derivatives to bovine serum albumin were obtained by a circular dichroic probe technique; The lipophilicity of the drugs, measured by a reversed-phase thin-layer technique using oleyl alcohol and methanol-water mixtures as the solvents, is expressed as RMw. The binding constants were of the same order of magnitude as the literature values, and the RMw values correlated well with log P octanol values from the literature. Correlations of log K with RMw were found to be more satisfactory when corrections for the state of ionization of the phenothiazines were made, the nonprotonated species accounting for the bulk of the binding. A better correlation was obtained when contributions from both species were taken into account. Similar correlations were attempted between RMw values and enzyme inhibitory activities of these phenothiazines taken from the literature.