  We derive new constraints on the products of explicitly R-parity violating
couplings $\lambda$ and $\lambda'$ in MSSM from searches for \mu-e conversion
in nuclei. We concentrate on the loop induced photonic coherent conversion
mode. For the combinations $|\lambda\lambda|$ which in \mu-e conversion can be
probed only at loop level our constraints are in many cases more stringent than
the previous ones due to the enhancement of the process by large
$\ln(m^2_f/m^2_{\tilde f}).$ For the combinations of $|\lambda'\lambda'|$ the
tree-level \mu-e conversion constraints are usually more restrictive than the
loop ones except for two cases which involve the third generation. With the
expected improvements in the experimental sensitivity, the \mu-e conversion
will become the most stringent test for all the involved combinations of
couplings.
