A set of 24 monoclonal antibodies (MABs) was produced against an outer envelope preparation from Leptospira interrogans serovar copenhageni. The MABs reacted in enzyme immunoassay with species-specific determinants of an antigen in the leptospiral outer envelope (OE) of pathogenic but not of saprophytic species of Leptospira. The MABs did not agglutinate whole leptospires, nor could they opsonise homologous leptospires for phagocytosis by mouse macrophages or protect new-born guinea-pigs against lethal infection. The MABs reacted by Western blotting with a 35 x 10(3)-mol-wt band in OE separated on SDS-polyacrylamide gels, and also reacted with other bands to a lesser extent. The determinants to which the MABs were directed were localised in the leptospiral OE by immunogold labelling techniques.