In this article we will present the Leibniz-Mycielski axiom (LM) of set theory (ZF) introduced several years ago by Jan Mycielski as an additional axiom of set theory. This new postulate formalizes the so-called Leibniz Law (LL) which states that there are no two distinct indiscernible objects. From the Ehrenfeucht-Mostowski theorem it follows that every theory which has an infinite model has a model with indiscernibles. The new LM axiom states that there are infinite models without indis-cernibles. These models are (...) called Leibnizian models of set theory. We will show that this additional axiom is equivalent to some choice principles within the axio-matic set theory. We will also indicate that this axiom is derivable from the axiom which states that all sets are ordinal definable (V=OD) within ZF. Finally, we will explain why the process of language skolemization implies the existence of indis-cernibles. In our considerations we will follow the ontological and epistemological paradigm of investigations. (shrink)