Interleukin-1 (IL-1 beta), tumour necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) were measured in serum from children with measles using an immunoradiometric assay. The IFN-gamma level was increased in 52 out of 54 patients in the acute phase of measles (less than 7 days of illness), and then declined to an undetectable level in the convalescent phase. Neither IL-1 nor TNF could be detected during the course of the illness. The mean serum IFN-gamma level was at its peak on day 4 and could be detected over a 7-day period after the onset of fever, coinciding with the febrile period (6.9 +/- 1.5 days). In the acute phase, the phytohaemagglutinin responses, absolute number of platelets, total lymphocyte counts, CD3+, CD4+, CD8+ cell counts and the CD4/8 ratio were depressed, while stab cell number and lactate dehydrogenase levels were higher than those in the convalescent phase. Using Spearman rank sum test, the IFN-gamma level was correlated negatively with the peripheral lymphocyte (P less than 0.01), CD3+ (P less than 0.05), CD4+ (P less than 0.05) cell counts and the CD4/8 ratio (P less than 0.05) and correlated positively with the stab cell count (P less than 0.01) but not with any other parameter. When the acute phase findings were compared between 28 complicated and 40 uncomplicated patients, the former were younger (P less than 0.01) and had higher maximum body temperature during the illness (P less than 0.05) than the latter, there was no difference in their IFN-gamma levels. These results show that endogenous IFN-gamma appears in the circulation during the acute febrile phase of measles, but does not contribute directly to any complication of the disease.