The effect of doping in CeNiSn has been studied by the measurements of electrical resistivity, Hall coefficient and magnetic susceptibility for single crystals of CeNi1-xTxSn (T=Co, Cu, and Pt) and Ce1-yLayNiSn (x,y = 0.01 and 0.05). All these impurities are found to increase the residual resistivity by several times up to 1 mΩ cm for x or y = 0.01, while for x or y = 0.05 the resistivities along the orthorhombic b and c axes saturate to values smaller than those for 0.01. Furthermore, the low-temperature increase in the Hall mobility of CeNiSn was found to be strongly suppressed in a similar way by all the impurities. These results indicate that residual carriers in CeNiSn with an anisotropic gap are immobilized by any impurity substituted either in the 4f site or the non-4f site.