Proteinoids are protein-like molecules formed inorganically from amino acids. Some theories of abiogenesis propose that proteinoids were a precursor to the first living cells.

The inorganic polymerization of amino acids into proteins through the formation of peptide bonds was thought to occur only at temperatures over 140°C. However, the biochemist S. W. Fox and his co-workers discovered that phosphoric acid acted as a catalyst for this reaction. They were able to form protein-like chains from a mixture of 18 common amino acids at only 70°C in the presence of phosphoric acid, and dubbed these protein-like chains protenoids. Fox later found proteinoids similar to those he had created in his laboratory in lava and cinders from Hawaiian volcanic vents and determined that the amino acids present polymerized due to the heat of escaping gases and lava. Other catalysts have since been found; one of them, amidinium carbodiimide , is formed in primitive Earth experiments and is effective in dilute aqueous solutions.

When present in certain concentrations in aqueous solutions, proteinoids form small structures called microspheres or protocells. This is due to the fact that some of the amino acids incorporated into proteinoid chains are more hydrophobic than others, and so proteinoids cluster together like droplets of oil in water. These structures exhibit many of the characteristics of cells:

1. a film-like outer wall.
2. osmotic swelling and shrinking.
3. budding.
4. binary fission (dividing into two daughter microspheres).
5. streaming movement of internal particles.

Microspheres are not themselves alive, but it is thought that they may have provided a cell-like compartment within which biochemistry could have become concentrated and protected from the outside environment during the process of chemical evolution.

Proteinoid microspheres are today being considered for use in pharmaceuticals, providing microscopic biodegradeable capsules in which to package drugs for delivery to a specific region in the patient's body.