AbstractPacking contacts are crystal artifacts, yet they make use of the same forces that govern specific recognition in protein‐protein complexes and oligomeric proteins. They provide examples of a nonspecific protein‐protein interaction which can be compared to biologically relevant ones. We evaluate the number and size of pairwise interfaces in 152 crystal forms where the asymmetric unit contains a monomeric protein. In those crystal forms that have no element of 2‐fold symmetry, we find that molecules form 8 to 10 pairwise interfaces. The total area of the surface buried on each molecule is large, up to 4400 Å2. Pairwise interfaces bury 200–1200 Å2, like interfaces generated at random in a computer simulation, and less than interfaces in protease‐inhibitor or antigen‐antibody complexes, which bury 1500 Å2 or more. Thus, specific contacts occurring in such complexes extend over a larger surface than nonspecific ones. In crystal forms with 2‐fold symmetry, pairwise interfaces are fewer and larger on average than in the absence of 2‐fold symmetry. Some bury 1500–2500 Å2, like interfaces in oligomeric proteins, and create “crystal oligomers” which may have formed in the solution before crystallizing. © 1995 Wiley‐Liss, Inc.