Several reports have suggested that asthmatic children may be particularly vulnerable to viral respiratory infections, in addition to this, abnormal cellular regulation has been suggested as an explanation for the increased IgE levels in atopy. Despite such evidence there have been few studies evaluating cellular immune function in asthmatic children. This study has examined in vivo and in vitro aspects of cellular immune function in twenty-six atopic asthmatic children and compared them with nineteen age-matched non-atopic control subjects. In vivo cellular immunity was assessed by quantitating the cutaneous delayed hypersensitivity (CDH) response to four ubiquitous antigens. Lymphocyte responsiveness to three polyclonal mitogens was measured in vitro. The serum IgE level was determined in all study subjects. The results indicate that CDH to Candida antigen is signifxicantly diminished among the asthmatic subjects and lessened to streptokinase/streptodornase. Lymphocyte mitogen response did not differ significantly between the study groups. No correlation could be shown between in vitro lymphocyte responsiveness and IgE level. These studies suggest that a subtle cellular immune defect may exist in asthmatic children and support the need for further studies of this question.