Secretory vesicles called trichocysts structurally show a body and a tip in Paramecium and a shaft and four arms in Pseudomicrothorax. Biochemical and immunological relationships between both trichocyst types were examined. Three polyclonal antibodies were produced against 3 groups of protein bands of isolated Pseudomicrothorax dubius trichocysts separated by one-dimensional gel electrophoresis: G 1 (30–31 kDa), G 2 (26–27 kDa) and G 3 (15–20 kDa). By indirect immunofluorescent binding assay, the three antisera label extended (i.e. discharged) and condensed (i.e. non-discharged) trichocysts of both Ps. dubius and Paramecium caudatum. By immunogold labeling on ultrathin sections, the three antisera label the trichocyst shaft of Ps. dubius and the trichocyst body of Pa. caudatum on both condensed and extended secretory vesicles. On two-dimensional immunoblots, remarkable antigenic similarities are shown by trichocysts of Ps. dubius, Paramecium tetraurelia nd6 and Pa. caudatum. Anti-G 1 serum detects proteins at 28–32 kDa, pI 5.0–5.7, in the three cells. Anti-G 3 serum labels one protein group at ~ 15–22 kDa and a second protein group at ~ 35–50 kDa in Ps. dubius, Pa. tetraurelia nd6 and Pa. caudatum. Anti-G 2 serum labels proteins at ~ 15–22 kDa, pI 4.7–5.2, in all three species. Anti-G 2 serum also detects bands at 24–28 kDa and 30–31 kDa, pI 4.7–6.1, in Ps. dubius. No equivalent M_r proteins are observed in either Paramecium spp. Comparisons of immunoblots of proteins of entire cells with and without ejectable trichocysts allowed identification of non-ejectable trichocyst proteins, some of which may be precursors localized within pretrichocysts. Such proteins are at 41–47 kDa in Ps. dubius, and at ~ 45 kDa in Paramecium.