The major cell envelope proteins of the gram-negative thermophilic eubacterium "Thermus thermophilus" gave an electrophoretical pattern characterized by two well-defined groups of bands. One of them showed up as four regularly spaced proteins (HMrPs) with Mrs higher than 310,000, a value corresponding to the smaller HMrP. The second one was formed by two proteins with Mrs of 100,000 (P100) and 84,000 (P84). HMrPs P100 and P84 were apparently located in the outer layer of the cell envelope, as indicated by their accessibility, in intact cells, to external lactoperoxidase and by their association, in fractionation experiments, with a high-density membrane fraction devoided of NADH-oxidase activity. Removal of Ca2+ unstabilized the HMrPs, which dissociated into P100 when heated at 80 to 85 degrees C in 10% (wt/vol) sodium dodecyl sulfate, indicating that HMrPs were oligomeric complexes of P100. In the presence of Ca2+, HMrPs were extremely stable, withstanding prolonged incubation in boiling 10% (wt/vol) sodium dodecyl sulfate-2% (vol/vol) beta-mercaptoethanol. Solubilization of P100 and HMrPs by detergents was severely constrained by interactions with the peptidoglycan layer of the cell envelope.