The morphogenesis of lambda proheads is governed by the products of at least four bacteriophage-coded genes (B, C, E and Nu3) and two host-coded genes (groES (mopB) and groEL (mopA)). Earlier genetic experiments indicated that the phenotypes of some of the groES- mutations could be suppressed by mutations in the groEL gene, suggesting an interaction between the two groE proteins in vivo (Tilly, K., and Georgopoulos, C. P. (1982) J. Bacteriol. 149, 1082-1088). The Mr 15,000 groES protein was overproduced and purified to homogeneity by monitoring its presence after polyacrylamide gel electrophoresis. Both gel filtration on an AcA34 sizing column and glycerol gradient centrifugation indicate that the groES protein possesses an oligomeric structure of Mr 80,000. In agreement, electron microscopic pictures of the purified groES protein show that it possesses a symmetrical ring-like structure. The sequence of the first five amino acids and the overall composition of the purified protein match those predicted by the nucleotide sequence of the groES gene. The following results implicate a physical association between the groES and groEL proteins in vitro. The groES protein inhibits the weak ATPase activity of the groEL protein, with a maximal effect seen at a 1:1 molar ratio; the two proteins cosediment during glycerol gradient centrifugation in the presence of ATP and Mg2+; and the groES protein binds specifically to a groEL-affinity column. These results help explain why mutations in either of the groE genes exhibit similar phenotypes with respect to both lambda and bacterial growth.