The license governing a distribution is included as part of that distribution.
The following license applies to the "jikesos" release of 7 December 1998 that included both the compiler and test suite: License I.
Beginning 1 February 1999, there are separate licenses for the compiler (jikes) and test suite (jikestst), which share the same text except for the name: "Jikes Compiler" and "Jikes Test Suite", respectively: License II.
Beginning 17 June 1999, we are using the IBM Public License Version 1.0. This applies to versions of the compiler starting with v0.52, and will be used when we next update the Jikes Test Suite and Jikes Parser Generator: IBM Public License Version 1.0 - Jikes Compiler