To study T cell and macrophage activity during measles, levels of interferon-gamma (IFN-gamma) and neopterin in plasma and cerebrospinal fluid (CSF) were measured. Plasma levels of IFN-gamma were elevated in measles (1.17 +/- 0.27) compared with healthy adults (0.13 +/- 0.06, P less than .05) and children (0.14 +/- 0.06, P less than .01). Plasma levels of neopterin were elevated in measles (32.5 +/- 2.7) compared with healthy adults (5.3 +/- 2.9, P less than .0001), healthy children (12.1 +/- 4.0, P less than .001), and children with other infectious diseases (20.6 +/- 4.0, P less than .02). IFN-gamma was increased in measles primarily during rash; neopterin remained elevated for several weeks. Levels of neopterin showed a significant positive correlation with plasma levels of soluble interleukin-2 receptor and soluble CD8, two other parameters of T cell activation. Children with measles complicated by pneumonia had higher levels of neopterin in serum than those with uncomplicated disease. Children with measles complicated by autoimmune encephalomyelitis had higher levels of neopterin in CSF than those with noninflammatory neurologic disease but lower than those with central nervous system infections. Thus, IFN-gamma seems to be produced in vivo during acute measles virus infection; deficiency of this lymphokine does not appear to correlate with increased susceptibility to secondary infections.