Immune responses are strongly influenced by the cytokines following antigenic stimulation. Distinct cytokine-producing T cell subsets are well known to play a major role in immune responses and to be differentially regulated during immunological disorders, although the characterization and quantification of the TH-1/TH-2 cytokine pattern in T cells remained not clearly defined. Expression of cytokines by T lymphocytes is a highly balanced process, involving stimulatory and inhibitory intracellular signaling pathways. The aim of this study was (1) to quantify the cytokine expression in T cells at the single cell level using optical imaging, (2) and to analyze the influence of cyclic AMP- dependent signal transduction pathway in the balance between the TH-1 and TH-2 cytokine profile. We attempted to study several cytokines (IL-2, IFN-(gamma) , IL-4, IL-10 and IL-13) in peripheral blood mononuclear cells. Cells were prestimulated in vitro using phytohemagglutinin and phorbol ester for 36h, and then further cultured for 8h in the presence of monensin. Cells were permeabilized and then simple-, double- or triple-labeled with the corresponding specific fluorescent monoclonal antibodies. The cell phenotype was also determined by analyzing the expression of each of CD4, CD8, CD45RO and CD45RA with the cytokine expression. Conventional images of cells were recorded with a Peltier- cooled CCD camera (B/W C5985, Hamamatsu photonics) through an inverted microscope equipped with epi-fluorescence (Diaphot 300, Nikon). Images were digitalized using an acquisition video interface (Oculus TCX Coreco) in 762 by 570 pixels coded in 8 bits (256 gray levels), and analyzed thereafter in an IBM PC computer based on an intel pentium processor with an adequate software (Visilog 4, Noesis). The first image processing step is the extraction of cell areas using an edge detection and a binary thresholding method. In order to reduce the background noise of fluorescence, we performed an opening procedure of the original image using a structuring element. The opened image was therefore subtracted from the original one, and the gray intensities were subsequently measured. Fluorescence intensities are mapped in MD representation using Matlab software. Consequently, quantitative comparative expression of intracellular cytokines and cell membrane markers was achieved. Using this technique, we showed that CD4+ and CD8+T lymphocytes expressed a large panel of cytokines, and that protein kinase A (PKA) activation pathway induced a polarization of activated human T cells to the TH-2 type profile. Data also showed different sensitivities of CD45 RO/CD45RA lymphocytes to the activation of PKA, thus suggesting the implication of memory CD4+- and CD8+-T cells in the T cell specific immune and inflammatory processes and their control by PKA activation pathway. Finally, this method represents a powerful tool for the detection and quantification of intracellular cytokine expression and the analysis of the functional properties of T lymphocytes during immune responses.