the species gluconacetobacterdiazotrophicus, herbaspirillum seropedicae and h. rubrisubalbicans are endophytic n2-fixing [diazotrophic] bacteria which colonise not only roots, but also the aerial tissue of sugar cane. however, the technique most commonly used to quantify the populations of these microbes in plants is by culturing serial dilutions of macerates of plant tissues in n free semi-solid media which are only semi-selective for the species/genera [the most probable number (mpn) technique] and each culture must be further subjected to several tests to identify the isolates at the species level. the use of species-specific polyclonal antibodies with the indirect elisa (enzyme-linked immunosorbent assay) can be an alternative which is rapid and specific to quantify these populations of bacteria. this study was performed to investigate the viability of adapting the indirect elisa technique to quantify individually the populations of these three species of diazotroph within the root and shoot tissues of sugarcane. the results showed that species-specific polyclonal antibodies could be obtained by purifying sera in protein-a columns which removed non-specific immuno-globulins. it was possible to quantify the three bacterial species in the brazilian sugarcane variety sp 70-1143 in numbers above 105 cells per g fresh weight in roots, rhizomes and leaves. the numbers of the different bacterial species evaluated using the elisa technique were found to be higher than when the same populations were evaluated using the mpn technique, reaching 1400 times greater for g. diazotrophicus and 225 times greater for herbaspirillum spp. these results constitute the first quantification of herbaspirillum using immunological techniques.