This article deals with the problem of when, given a collection
$\mathcal {C}$
of weakly compact operators between separable Banach spaces, there exists a separable reflexive Banach space Z with a Schauder basis so that every element in
$\mathcal {C}$
factors through Z (or through a subspace of Z). In particular, we show that there exists a reflexive space Z with a Schauder basis so that for each separable Banach space X, each weakly compact operator from X to
$L_1[0,1]$
factors through Z.
We also prove the following descriptive set theoretical result: Let
$\mathcal {L}$
be the standard Borel space of bounded operators between separable Banach spaces. We show that if
$\mathcal {B}$
is a Borel subset of weakly compact operators between Banach spaces with separable duals, then for
$A \in \mathcal {B}$
, the assignment
$A \to A^*$
can be realised by a Borel map
$\mathcal {B}\to \mathcal {L}$
.