Specimens obtained by broncho-alveolar lavage (LBA) allow for an examination for viral antigens by a direct technique using monoclonal antibodies and by culture, and the level of IgM and IgG antibodies by the ELISA technique. LBA is used in the screening of immuno-depressed subjects and in the search for an aetiology in overt cases of interstitial pneumonia. Ninety-six specimens of LBA from 76 patients were analysed [38 had immunological deficits of whom 14 had haematological disorders, 15 were transplant cases and there were 9 in the miscellaneous group (group A). There were 38 pneumonias occurring in pre-existing chronic disorders (group B)]. Twenty respiratory viral infections were identified (26.3%): 17 by direct examination or culture (22.4%), 3 by serology. CMV was identified in 14 cases, HSV1 in 4 cases, VZV and VRS in one case each. In the haematological cases there was a positivity of 28.6%, 66% in the transplants (100% for heart transplants), and from 11 to 13.2% in other groups. Ten patients in 56 (17.8%) possessed anti CMV IgG antibodies (the mean level was 745) and 5 had IgM (mean level 18) (9%). One case of pneumonia due to VZV with IgM and IgG in the LBA was reported. In summary, a direct examination by LBA is positive in a quarter of the cases and a culture in three quarters. The conventional methods failed here, showing the frequency of purely localised infections in the respiratory tract.(ABSTRACT TRUNCATED AT 250 WORDS)