  We report on measurements of the temperature dependence of the magnetic
penetration depth of a very high quality single crystal of nonmagnetic
superconductor LaPt3Si without inversion symmetry. The results are compared
with those previously reported for the isostructural antiferromagnetic
superconductor CePt3Si. At low temperatures, the penetration depth follows a
BCS exponential behavior that implies an isotropic energy gap in LaPt3Si, in
contrast to a linear response that indicates line nodes in CePt3Si. These line
nodes have been argued to be protected by symmetry or accidentally generated by
parity mixing. The present results provide support for the viewpoint that
parity mixing alone does not seem to lead to unconventionality in CePt3Si and
that it requires the antiferromagnetic order to be included.
