  We present a relativistic formulation of noncommutative mechanics were the
object of noncommutativity $\theta^{\mu\nu}$ is considered as an independent
quantity. Its canonical conjugate momentum is also introduced, what permits to
obtain an explicit form for the generators of the Lorentz group in the
noncommutative case. The theory, which is invariant under reparametrization,
generalizes recent nonrelativistic results. Free noncommutative bosonic
particles satisfy an extended Klein-Gordon equation depending on two
parameters.
