The effect of Brij 78, a nonionic surfactant, on the interactions between cationic minoxidil (MXD) microparticles and sodium lauryl ether sulfate (SLES) in an aqueous phase was investigated. Cationic MXD microparticles were prepared by high pressure homogenization, where distearyldimethylammonium chloride (DSDMAC) was used as a cationic dispersant. The positive surface charge of MXD microparticles was neutralized at the equimolar ratio of SLES/DSDMAC and became negative in the presence of excess amount of SLES, whether Brij 78 was included in the MXD suspensions or not. On the other hand, when nonionic surfactant was included in the suspension so that the molar ratio of Brij 78/DSDMAC was 0.063 : 1, the suspension of the cationic microparticles was stable in terms of turbidity change even in the presence of excess amount of SLES. It means that while electrostatic interactions between DSDMAC and SLES were occurring, the aggregation of cationic MXD microparticles was prevented with the aid of Brij 78. Due to the cationic microparticle-stabilizing effect of Brij 78 in the presence of the anionic surfactant, the substantivity of the cationic MXD microparticles having Brij 78 was about two times higher than that of the microparticles without the nonionic surfactant.