Fourteen healthy children and nine children with various immune-deficiency states were tested for the ability of their lymphocytes to release skin reactive factor (SRF). All normal controls and those whose lymphocytes underwent blastic stimulations with PHA or PPD show SRF activity as tested in the guinea pig dermis. Six children, one of them with generalized BCG infection, and five whose cellular immunity was decreased by immunosuppression, steroid therapy and/or infectious viral agent, were unable to release SRF. This was in correlation with impaired reaction to intradermal PHA or absent blastic transformation, or no detectable activity of macrophage inhibition factor (MIF). It is suggested that the SRF assay for cell-mediated immunity results from a similar mechanism as that indicated by the MIF assay and is much simpler in nature.