DNA of Leptospira interrogans sv. lai strain 017 was labelled with digoxigenin or alpha 32P or biotin and used as probes to detect DNA of leptospires. Probes labelled with digoxigenin were able to detect 0.1-1 pg of homologous DNA and 10(2) of L. interrogans sv. lai strain 017. Probes alpha 32P and biotin detected 1 pg and 10 pg of homologous DNA and 10(3), 5 x 10(3) of L. interrogans sv. lai strain 017 respectively. These three probes couldn't detect L. biflexa sv. patoc strain Patoc I, L. illini strain 3055, Escherichia coli. Bacillus aerogenes capsulalus, Salmonella anatis, K-DNA of Leishmania, and of human WBC. Comparison of digoxigenin-, alpha 32P- and Biotin-labelled probes in the dot-blot hybridization assay on different serogroup sv. of leptospires revealed that digoxigenin-labelled probes were more sensitive than alpha 32P and biotin-labelled probes. The results indicate that digoxigenin-labelled probes DNA can be used for detection of leptospires in field and clinic. We also report procedures in situ hybridization with leptospira in tissues smear and plasma sediment of an experimentally infected guinea pig. It offers the advantage of recognizable of leptospiral morphology in combination with a hybridization signals. The results indicate that digoxigenin-labelled DNA probes of 017 strain might provide tool for routine diagnosis and classification in cases of leptospiral interrogans infection.