Fourteen human prolactinomas, four of which received bromocriptine treatment, were studied by immunoelectron microscopy to observe the intracellular localization of prolactin (PRL). In the untreated cases two different patterns of PRL localization were noted. In the first type (Type I) PRL was seen in Golgi saccules, single stacks of rough endoplasmic reticulum (RER), and a few secretory granules and in the second type (Type II) it was present in the predominant parallel arrays of RER. Type II cells were observed more frequently in the cases with higher serum prolactin levels. In the treated cases with normalized serum prolactin levels, the tumor cells showed accumulation of secretory granules. This finding suggested secretory inhibition. RER and Golgi saccules were inconspicuous in these cells and this might indicate decreased production of PRL. In a case which was resistant to the bromocriptine treatment, most tumor cells were similar in ultrastructural localization of PRL to those cells in the untreated cases and this might suggest the lack of dopamine receptor.