Comparable diets were found to modulate levels of specific and natural humoral immunity in different manners over two generations of genetically selected hens for a high or low Ab response. These diets were based on ingredients that were grown organically (diet A) or conventionally (diet B). Here we report the effects of these diets on cellular immune parameters such as monocyte reactivity measured by NO production, proliferation of whole blood leucocytes and PBMC with the T- and B-cell mitogens, ConA and LPS respectively. Furthermore we measured the in vitro modulatory effects of water soluble extracts of the two diets on T-cell proliferation of whole blood cultures. In both generations a feed change enhanced monocyte reactivity in all birds with the high line birds being most sensitive. Whole blood assays showed the most pronounced diet effects on T-cell reactivity. The low line birds of the first generation showed the greatest effect, but in the second generation all lines were affected by the diets. In the PBMC the greatest effects were found in the control values, with the effects differing in each generation. The present results together with the results found in the whole blood cultures, suggest dietary effects on the intrinsic reactivity of peripheral lymphocytes as well as in vivo effects, the first with quickly measurable effects and the last activity reflected in our later measurements. These results suggest that a feed change with only minor nutritional differences will induce immunomodulatory effects, and each diet has a unique effect on cellular parameters of innate and adaptive immunity.