To study T cell and macrophage activity during measles, levels of interferon-γ (IFN-γ) and neopterin in plasma and cerebrospinal fluid (CSF) were measured. Plasma levels of IFN-γ were elevated in measles (1.17 ±0.27)compared with healthy adults (0.13 ± 0.06, P<.05)and children (0.14 ± 0.06, P <.01).Plasma levelsof neopterin were elevated in measles (32.5 ± 2.7) compared with healthy adults (5.3 ± 2.9, P <.0001), healthy children (12.1 ± 4.0, P <.001), and children with other infectious diseases (20.6 ± 4.0, P <.02). IFN-γ was increased in measles primarily during rash; neopterln remained elevatedfor several weeks. Levels of neopterln showed a significant positive correlation with plasma levels of soluble interleukin-2 receptor and soluble CDS, 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-γ seems to be produced in vivo during acute measles virus infection; deficiency of this Iymphokine does not appear to correlate with increased susceptibility to secondary infections. © 1990, University of Chicago. All rights reserved.