The numerous infections of microsporidia which have been diagnosed in patients with AIDS have revealed the potential of these organisms for establishing themselves when the immune status of the host is compromised. Two species of Encephalitozoon, E. cuniculi and E. hellem, have been diagnosed in man, the former infecting a variety of tissues, the latter restricted to the corneal and conjunctival epithelia. These species are morphologically indistinguishable even at the ultrastructural level but can be separated biochemically. Two human sera were found to react with equal intensity in the ELISA on spores of E. cuniculi and E. hellem purified from in vitro cultures, and gave similar binding patterns in Western blots on SDS-PAGE protein profiles of the two species. This has raised questions about the identity of Encephalitozoon infections diagnosed previously in man. The diagnosis of Enterocytozoon bieneusi, which infects the intestinal enterocytes of AIDS patients and is associated with chronic diarrhoea, requires observation of smears or sections of biopsies or specialist observation of stool preparations. In vitro cultures, which would facilitate the raising of specific antisera, have proved difficult to establish. In vitro and in vivo systems for assaying drugs for microsporidia have revealed that albendazole has a marked effect on parasite numbers and morphology but does not eliminate infection, which resurges when drug pressure is removed.