Peripheral blood mononuclear leukocytes (PBML) from 21 human donors seropositive for herpes simplex virus (HSV) type 1, and from seven seronegative donors were employed to produce interferon (IFN) in response to stimulation by HSV and phytohemagglutinin (PHA). Cryopreserved PBML produced titers of IFN, including immunespecific IFN, that were not significantly different from titers produced by freshly prepared PBML. In addition, cryopreserved PBML stored under conditions designed to simulate inter-institutional exchange of specimens produced quantities of IFN that did not vary significantly from quantities of IFN produced by cryopreserved cells that were stored under conventional conditions. Production of immunespecific IFN by cryopreserved cells represents an example of a measurement of cell-mediated immunity that is analogous in reliability and flexibility to currently available measurements of humoral immune responses.