Non-caseating epithelioid granulomas have been induced in rats by the subcutaneous injection of BCG vaccine and their cellular disaggregation to yield isolated epithelioid cells has been achieved using collagenase. Ultrastructurally epithelioid cells in both intact granulomas and disaggregated cell suspensions showed a spectrum of appearances ranging from cells with conspicuous rough surfaced endoplasmic reticulum ("plasmacytoid epithelioid cells") to cells containing numerous cytoplasmic membrane-bound vesicles ("vesicular epithelioid cells"). Numerous intermediate forms were present. Endocytosed material was inconspicuous. Investigation of disaggregated 4, 10 and 28 day lesions showed that the proportion of vesicular epithelioid cells increased with maturation of the granulomas. Nevertheless a full spectrum of epithelioid cell morphology was present as early as 4 days. It is suggested that mononuclear phagocyte cells entering the granuloma transform into vesicular epithelioid cells via an intermediate plasmacytoid stage. The successful isolation of viable epithelioid cells from granulomas may allow the function of these cells to be evaluated further.