Five hormones in lymphatic cells from the spleen and thymus of newborn rats were measured using flow cytometry and confocal microscopic immunofluorescence. Immunoreactive histamine, serotonin and triiodothyronine (T3) were present in the cells. However, growth hormone and insulin were not observed, except for low levels of insulin in a few spleen cells. Serotonin content was modest compared to the histamine and T3 levels. The localization of each hormone in the cells was different. The importance of these observations in the light of functional data is discussed.