  A system of first-order differential equations for a particle with nonzero
mass and spin $S = 1$ is constructed. As distinct from the Proca-Duffin-Kemmer
(PDK) equations, the system has the form of the dynamical equation
$i\hbar\partial_t\hat{\Psi}=\hat{\mathrm{H}}\hat{\Psi}$ (with constraints) with
a Hamiltonian linear in momentum. The six-component wave function yields the
positive-definite probability density $\hat{\Psi}^{\dagger}\hat{\Psi}\geq 0$.
The system of equations has much in common with the Dirac and Maxwell
equations.
