Delayed hypersensitivity (dh) skin test reactivity to a somatic antigen of Haemophilus influenzae was studied in 21 patients with unexplained, chronically relapsing, purulent upper respiratory tract infections. Only 2 showed a dh reactivity comparable to that of healthy controls. A majority--15 patients--had a defective dh response, whereas 4 showed exaggerated reactivity leading to necrosis of the test site and general feelings of malaise. Not only was the dh reactivity to somatic H. influenzae antigen affected, but also that to streptokinase/streptodornase and candidal antigen in most cases, though to a lesser extent. Skin test reactivity to the mitogen PHA was normal as were the dh skin test reactivities in 4 out of 5 control patients with mucous atopic rhinitis/sinusitis and 2 cases of nasal suppuration due to disturbed mucociliary transport. Delayed hypersensitivity skin test disorders were associated with elevated ratios of OKT4 + /OKT8 + peripheral lymphoid cells. In addition a high incidence of atopy and thyroid autoimmunity was evident in patients as well as in their first-degree relatives. A negative lymphocyte proliferative response to somatic H. influenzae antigen was found in 3 of our patients. These results suggest that unexplained, chronically relapsing upper respiratory tract infections might be based on restricted T-cell defects to H. influenzae, streptococcal, and candidal antigens. Such defects are reminiscent of the T-cell immune disorders to fungi playing a role in some cases of chronic mucocutaneous candidiasis.