A better vaccine than the existing ones against contagious bovine pleuropneumonia (CBPP) caused by Mycoplasma mycoides subsp. mycoides small colony type (MmmSC) would improve the chances for eradication of CBPP. In such an effort, immunostimulating complexes (ISCOMS) have been prepared from the whole detergent-solubilized cells of MmmSC and characterized biochemically and immunologically. The most efficient detergent for solubilization of the mycoplasma was MEGA-10 which yielded a high recovery of proteins in the ISCOMS. The ISCOMS showed the typical cage-like structure by EM and sedimented as 19S by sucrose gradient centrifugation. The protein pattern of the ISCOMS, analyzed in SDS-PAGE, revealed a great number of bands distributed along the gel as high and low molecular weight polypeptides. The Western blot developed with a serum from a CBPP infected animal detected a reduced number of polypeptides. In samples from whole mycoplasma cells and in ISCOMS, lectin blots revealed more than 20 carbohydrate structures. The ISCOMS induced a strong primary antibody response in mice measured by ELISA and the boost resulted in a 6-fold increase of the serum antibody response. The IgG response was distributed into various IgG subclasses with high IgG1, IgG2a and IgG2b titres while the IgG3 response was low. In cattle the ISCOM vaccine induced strong primary and long lasting secondary antibody responses of similar magnitudes as those of naturally infected animals as recorded by ELISA which persisted more than a year. IgG response was equally distributed in IgG1 and IgG2 subclasses. Also a cell-mediated immune response measured by proliferation assay was induced by low dose of ISCOMS. In the growth inhibition test, sera from vaccinated cattle readily inhibited colony growth already after the first immunization.