First-principles methods are employed to study the ground-state properties of δ-Pu-based alloys. The calculations show that an alloy component larger than δ-Pu has a stabilizing effect. Detailed calculations have been performed for the δ-Pu1−cAmc system. Calculated density of Pu-Am alloys agrees well with the experimental data. The paramagnetic → antiferromagnetic transition temperature (Tc ) of δ-Pu100−cAmc alloys is calculated by the Monte-Carlo technique. By introducing Am into the system, one could lower Tc from 548 K (pure Pu) to 372 K (Pu70Am30). We also found that, contrary to pure Pu where this transition destabilizes δ-phase, Pu3Am compound remains stable in the antiferromagnetic phase that correlates with the recent discovery of the Curie-Weiss behavior of δ-Pu100−cAmc alloys at c ≥ 24 at. %.