Using a magnetic field we can induce nucleation in superfluid He-3 both ways across the A-B phase boundary, at temperatures below 0.2T(c). The nucleation takes place in a quasiparticle black-body radiator in a variable magnetic field profile and is detected from the associated temperature jump. Before A-phase will nucleate, the parent B-phase must be overmagnetised by a history-dependent amount. Similarly, B-phase nucleation requires a degree of undermagnetisation. However, the transition is asymmetric and the history dependences are quite different. (C) 2000 Elsevier Science B.V. All rights reserved.