Immune responses in 24 children with acute measles (AM) were compared with those in 20 children who had chronic pulmonary complications (CPMC) following measles. The immuno-suppressive effects of acute measles were extensive: total white cells were reduced and this reduction was accounted for entirely by lymphopenia which was equally expressed among the major lymphocyte sub-populations studied; the function of 'T' cells, assessed by radio-isotope incorporation into phytohaemagglutin (PHA) transformed lymphocytes and delayed skin hypersensitivity (DHR) to dinitrochlorobenzene (DNCB), was depressed. Serum IgA was reduced in AM patients. In contrast there was a relative sparing of the measured indices of immunity in patients with chronic post-measles chest disease, with the major defect being an impaired DHR to DNCB. There were minor alterations of complement components in both groups of patients.