  We show that adding a random real number destroys a large fragment of
Martin's axiom, namely Martin's axiom for partial orders that have
precalibre-$\aleph_1$, thus answering an old question of J. Roitman [9]. We
also answer a question of J. Steprans and S. Watson [13] by showing that, by a
forcing that preserves cardinals, one can destroy the precalibre-$\aleph_1$
property of a partial ordering while preserving its ccc-ness.
