The equation of motion of a quantum-mechanical two-particle system is solved without approximation in an expansion in Planck's constant. First-order quantum corrections comprising all contributions to the coefficient of ℏ2 in the expansion are derived for the differential scattering cross section and the kinetic-theory transport coefficients. Numerical results are obtained for a simple model of molecular interactions, and lack of agreement is found with work based on the WKB solution of the Schrödinger equation.