Inclusion Criteria:

          -  Participants with documented diagnosis of intermediate or high-risk primary
             Myelofibrosis, post polycythemia Vera Myelofibrosis or post-essential thrombocythemia
             myelofibrosis

          -  Participant must be ineligible or unwilling to undergo stem cell transplantation at
             time of study entry

          -  ECOG of 0,1, or 2.

          -  Participant must have either received prior treatment with ruxolitinib OR another
             JAK-2 inhibitor therapy OR must not have received any prior treatment with JAK-2
             inhibitor.

          -  Participant has splenomegaly as defined in the protocol.

          -  Participant must meet the laboratory parameters (adequate bone marrow, renal and
             hepatic function) as defined in the protocol.

        Exclusion Criteria:

          -  Splenic irradiation within 6 months prior to screening, or prior splenectomy.

          -  Leukemic transformation (> 10% blasts in peripheral blood or bone marrow biopsy).

          -  Participant is currently on medications that interfere with coagulation (including
             warfarin) or platelet function with the exception of low dose aspirin (up to 100 mg)
             and Low-molecular-weight heparin.

          -  Prior therapy with a BH3 mimetic compound.

          -  Participant has received strong or moderate CYP3A inhibitors within 14 days prior to
             the administration of the first dose of navitoclax.