Determinations of in vitro proliferative and secretory activities of peripheral blood cells are used widely for research in clinical immunology but, to our knowledge, have not been evaluated as to their power to reflect in vivo activities quantitatively. Here, we addressed this question by quantitatively correlating the in vitro secretion of interleukin (IL)-5 by peripheral blood cells to the in vivo activity of IL-5 as reflected by peripheral-blood eosinophil counts. Studying 458 humans exposed to transmission of the nematode Onchocerca volvulus, IL-5 was measured in the supernatants of 0.02-ml whole-blood cells cultured in the presence of O. volvulus extract or mitogen. O. volvulus-reactive IL-5 secretion was correlated significantly to blood eosinophilia in a quantitative manner explaining 15.1% (95% CI 8.3-19.9%) of the variability of eosinophil counts. Interestingly, correlations were obtained only if parasite counts were included in the calculation using multiple regression analysis. The results show that in vitro assays of minute amounts of blood lymphocytes may quantitatively reflect activities of the entire lymphocyte population in vivo.