For practical purposes, kinetic hydrate inhibitors must perform predictably in the presence of oil as well as saline and high driving forces, but such deterministic behavior is rarely achieved. Here, we evaluated two biological inhibitors, type I and type III antifreeze proteins (AFPs I and III), under these exacting conditions using a double high-pressure crystallizer apparatus and additionally assayed using high-pressure micro-differential scanning calorimetry. The two AFP types behaved somewhat differently under these environmental conditions. The addition of AFP I reduced natural gas hydrate induction time, whereas AFP III had no impact on hydrate crystal nucleation. Nonetheless, for both AFPs, gas hydrate growth was significantly inhibited to similar to 50% of that found in control experiments. Once hydrate had formed, decomposition was slower and started later. Thus, gas hydrates formed in the presence of APF I and III appeared to remain stable outside the hydrate stable zone, an observation that has also been noted for other inhibitors. Our observations have potential implications for the use of biological inhibitors under subsea pipeline conditions.