It is shown that the alternative Klein-Gordon equation with positive definite probability density proposed in a letter by M.D. Kostin does not meet the requirements of relativistic (quantum) field theory and therefore does not allow for a meaningful physical interpretation. PACS code: 11.10.-z Keywords: Klein-Gordon equation, relativistic field theory. 2 The alternative formulation of the Klein-Gordon equation [1, 2] proposed by M.D. Kostin reads [3] i¯h ∂φ ∂t = +mc2φ + c ( ˆ⃗p ⃗ ψ) (1) i¯h ∂ ⃗ ψ ∂t = −mc2 ⃗ ψ + c ˆ ⃗pφ, (2) where φ(⃗r,t) and ⃗ ψ(⃗r,t) are ’scalar ’ and ’vector ’ probability amplitudes, respectively, and ˆ ⃗p = −i¯h ⃗ ∇. Defining the probability density P = φ ∗ φ + ( ⃗ ψ ∗ ⃗ ψ) (3) and the probability current density one readily derives the probability conservation equation ⃗S = c(φ ∗ ⃗ ψ + φ ⃗ ψ