  The concept of {\it active gravitational mass}, its definition and its
relation with the sources of a gravitational field, was clearly established by
Tolman in 1934. On the contrary, and surprisingly in our opinion, the concept
of {\it proper mass} has remained obscure in General relativity. We compare a
new definition to an apparent more obvious one and discuss how each choice
modifies the ratio of active to proper gravitational mass.
