  We report measurements of the magnetic penetration depth \lambda_m(T) in the
presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals.
Warming, after magnetic field is applied to a zero-field cooled sample, results
in a non-monotonic \lambda_m(T), which does not coincide with a curve obtained
upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the
possible relation of our results to the vortex decoupling, unbinding, and
dimensional crossover.
