H‐2blymphocytes were sensitized against H‐2dalloantigens by mixed lymphocyte culture reaction (MLR) and incubated with H‐2dtarget cells in the presence of prednisolone or without prednisolone (control). The interaction between lymphoid cells and target cells was stopped by fixation with glutaraldehyde. Changes in cell size, in numerical density of the endoplasmic reticulum (ER) and the Golgi apparatus (GA) as well as in areal density of the lysosomes and the mitochondria in the lymphoblasts during the first four hours of interaction with target cells were measured with a Hewlett Packard digitizer. The cell size showed no significant change within the first hour of interaction with target cells, but during the second hour a minor increase in cell diameter was observed whereas the diameter of the control lymphocytes increased a great deal during the first half hour as well as from the second to the third hour. The numerical density of the ER decreased throughout the four hour observation period, whereas the control lymphocytes exhibited a decrease in the numerical density of ER within the first hour after which the density increased. During the first hour of interaction with target cells the numerical density of the GA decreased and there was no increase during the rest of the observation period, whereas in the control lymphocytes the density of GA increased from the second to the third hour of interaction with target cells. There was no lysosome formation, and no significant change in the areal density of mitochondria during the four hour observation period, whereas the density of lysosomes as well as mitochondria increased in the control lympho