Fixation of rabbit spermatozoa with glutaraldehyde containing either malachite green or pyronine demonstrated a material which has not been previously described. In ejaculated spermatozoa, this malachite green and pyronine-affinity material (MGP-AM) appeared as a compact mass localized between the nuclear envelope and dense lamina of the posterior portion of the postacrosomal region. MGP-AM was absent in testicular spermatozoa and gradually accumulated during passage of spermatozoa through the epididymis, suggesting a possible correlation between the accumulation of this material and maturation of the spermatozoa. Among eighteen other cationic dyes tested, only methyl violet 2B could preserve this material. Similar MGP-AM was demonstrated in the spermatozoa of other species (human, rat, mouse, mongoose and hamster) within: subacrosomal space and nuclear vacuoles (human), redundant nuclear envelopes (human and mongoose), middle-piece mitochondria (human and hamster), and between nuclear envelope and dense lamina of the postacrosomal region (hamster, mouse and rat). The possible mechanisms by which MGP-AM is preserved by this technique are discussed.