1. Introduction: Pneumocystis carinii pneumonia(PCP) in immunocompetent hostsPneumocystis carinii is an opportunistic fungalagent that causes pneumonia in immunocomprom-ised patients, specially those with AIDS. Althoughthe complex host-parasite relationship is still incom-pletely understood, extensive data pertaining mostlyto immunocompromised patients as well as immuno-compromised animal models are available from in-vestigators both in the USA and in Europe.As opposed to the immunocompromised patients,data pertaining to the host-parasite interaction arescarce in immunocompetent individuals. An interest-ing ¢nding in these individuals was a lack of detect-able Pneumocystis DNA in bronchoscopic alveolarlavage samples [1]. Studies using both monoclonalantibodies and the more sensitive techniques ofPCR have failed to reveal evidence of Pneumocystiscolonisation in lung tissue obtained at post mortemor in broncho-alveolar lavage £uid from immuno-competent individuals [2^4]. Pneumocystis has beenthought to be acquired early in childhood. This pri-mary infection is thought to result in mild disease,which is seldom diagnosed or treated. Very few stud-ies have been conducted on the primary infection,although serological data have demonstrated an im-munological response in children to Pneumocystis atan early age [1,5,6]. Besides these observations, verylittle is known about the primary infection. The en-vironmental source(s) of the aetiologic agent is un-known and the infective stage of the parasite seemsnot to persist in the lungs. Thus, a possible linkbetween the primary infection, a subsequent pneu-mocystosis, and the host immune response has notbeen elucidated.Despite eiorts on the part of several investigators,a reliable in vitro cultivation system remains unavail-able and this has hampered the investigation of thebasic molecular and cellular biology of P. carinii.Therefore, the understanding of the interaction ofthe parasite with the host has been investigatedonly in animal models, which remain a very valuable