Protein:protein interactions and heterodimeric complexes are central to the regulation of apoptosis, a form of programmed cell death. The study of the formation of these heterodimeric complexes in endogenous contexts, and how they change in response to different stimuli has historically been difficult to study. Current methods to study these complexes use fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer (FLIM-FRET) and fluorescence correlation microscopy, both requiring over-expression of tagged proteins. We have developed multiple antibodies for two of these apoptotic heterodimer complexes (BCL2:BIM and MCL1:BIM) that can be used to analyse heterodimer levels in bulk endogenous cell samples via immunoprecipitation, and in single cells via flow cytometry. Characterisation of these antibodies by SPR and crystallography has revealed some interesting features. While antibodies for MCL1:BIM interact with both components of the heterodimer, two distinct conformational epitopes on BCL2:BIM show remarkable specificity for the heterodimeric complex, yet only interact directly with BCL2.