Micronemes of cystozoites of Sarcocystis muris were isolated after subcellular fractionation and used for immunization of BALB/C mice. After spleen cells of immunized mice were fused with SP20 myeloma cells, ten different monoclonal antibodies (mAbs) were isolated. These antibodies reacted with antigens whose molecular weight ranged from 16 to greater than 90 kDa. Six mAbs recognized granules of 150-400 nm that were located in the vicinity of the Golgi complex but were not identical with dense granules. Two mAbs (2A3, 3A8) were specific for micronemes of cystozoites as demonstrated by immunoelectron microscopy. However, these antibodies also recognized the population of granules near the Golgi complex. Cross-reactivity between micronemes and a dense granule population has not previously been reported. Host cells that had been contacted by cystozoites showed patchy fluorescence when probed with mAb 2A3. This suggests that microneme antigens could be transferred to the host-cell surface during parasite-host cell interactions.