The changes of myospherulosis were observed in masses from the buttocks of three patients. Ultrastructural examination of parent bodies and spherules from two patients revealed some spherules to contain dense bodies and filaments. Spherules had a double electron dense wall: the outer layer had a thickness corresponding to that of a cell membrane. Incubation of packed red blood cells with tetracycline ointment produced a similar electron dense deposit on the inner aspect of the cell membrane of the erythrocytes. No filaments or parent bodies were seen. Our findings support the hypothesis that myospherulosis represents altered red blood cells. The filaments seen in our patients may represent polymers of haemoglobin. It is suggested that parent bodies may be derived from histiocytes.