The Korringa-Kohn-Rostoker Green's function method within the coherent-potential approximation was applied to calculate the electronic properties of recently synthesised semi-Heusler TiCo_1-xNi_xSb solid solution. As evidenced parallel from electrical resistivity measurements, the calculations confirm a semiconductor type behaviour for TiCoSb compound and a metallic type one for alloys containing Ni. The electronic conductivity was found to originate from the d-electrons of Ti, Co and Ni, mainly.