Myotonic dystrophy (MyD), an autosomal dominant neuromuscular disease with multisystem abnormalities, is associated with hypercatabolism of IgG. The hypercatabolism is not related to structural abnormalities of the IgG molecule in MyD but appears to be due to a derangement of the serum IgG concentration-fractional catabolic rate relationship. Since the catabolic pattern of IgG is governed by the Fc portion of the molecule, the possibility of Fc receptor dysfunction in MyD has been explored. We have observed that although MyD patients have normal numbers of Fc receptor bearing leucocytes in their peripheral blood, MyD monocytes express significantly (P less than 0.02) greater numbers of Fc receptors (47.9 +/- 21.2 X 10(3) receptors/monocyte) than do monocytes of healthy subjects (29.1 +/- 9.6 X 10(3) receptors/monocyte). The mean affinity constants of the Fc receptors was lower in the MyD group (1.5 +/- 0.7 X 10(8)/M) than the normal control group (2.4 +/- 0.9 X 10(8)/M) but this difference was not statistically significant. MyD monocytes showed a propensity to shed Fc receptors in culture at 37 degrees C whereas no significant shedding was observed with control monocytes. Thus MyD monocytes may shed Fc receptors at physiological temperatures but at the same time express more receptors per cell than normal monocytes. This suggests that MyD monocytes may have an abnormally high turn-over of Fc receptors.