We optimized a whole blood intracellular cytokine assay to quantitate the frequency of specific CD4+ and CD8+ T cells in small volumes of whole blood from infants from developing countries. The assay is performed in two steps. First, whole blood is stimulated in the presence of specific antigens for 6-18 h, ending with cryopreservation of fixed white cells. These stimulation steps were specifically adapted to be practical and reliable in a rural, developing country field setting. Later, in a more resourceful setting, interferon-gamma producing CD4+ or CD8+ T cells are detected by flow cytometry. The assay proved sensitive and specific for detecting mycobacteria-specific immunity 10 weeks after Bacillus Calmette-Guerin (BCG) vaccination of newborns from a rural field site.