The contribution of the martensitic transformation to the overall stress-strain response of a multiphase steel assisted by a transformation- induced plasticity effect is analyzed in detail. A recently-developed multiscale transformation model is combined with a plasticity model to simulate the response of a three-dimensional grain of retained austenite embedded in a ferrite-based matrix. Results show that the effective hardening behavior of the material depends strongly on the grain orientation and, to a lesser extent, on the grain size