The outer membrane proteins of seven reference strains of pathogenic Leptospira (L. alstoni serovar grippotyphosa, L. borgpetersenii serovar hardjo, and L. interrogans serovars autumnalis, bratislava, canicola, icterohaemorrhagiae, and pomona) were investigated to identify common surface-exposed outer membrane proteins. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sodium-N-lauroylsarcosinate-insoluble outer membrane enriched fractions of the reference serovars and two field isolates of serovars hardjo and pomona revealed six common protein bands with approximate molecular masses of 77, 66, 42, 35.5, 24, and 18 kDa. At times the 35.5 kDa endoflagellar band resolved into two distinct bands, 35.5 kDa and 34 kDa. Immunoblotting of the same fractions using rabbit leptospiral antibodies showed six bands to be common (66, 59.5, 44, 42, 35.5, and 18 kDa). The 44 kDa band stained poorly with Coomassie blue but prominently by immunoblotting. Four reference strains (serovars bratislava, canicola, icterohaemorrhagiae, pomona), and two field isolates of serovar pomona and one of serovar bratislava were grown in low iron media to which the iron chelators 2,2'-dipyridyl or ethylenediaminehydroxyphenylacetic acid were added. No iron-dependent expression of outer membrane proteins was observed. The only difference observed between the outer membrane proteins when reference serovars of canicola or pomona were grown in dialysis bags in the peritoneum of swine or in vitro was the loss of the 77 kDa band from in vivo grown organisms. Treatment of whole leptospires with proteinase K did not remove the 77, 66, 59.5, or 42 kDa protein; these proteins may not be surface expressed or are inaccessible to the proteinase K. The 44 kDa band could not be evaluated by this method and the 18 kDa band was proteinase K resistant.