Cyclic hematopoiesis (CH), also called cyclic neutropenia, is an inherited disorder known to occur in both humans and gray collie dogs. Previous reports have provided ample evidence to suggest that lymphocyte activity and regulatory mechanisms may be abnormal in CH. The present study examined the lymphocyte populations of several lymphoid compartments of gray collie dogs. The percentage of B lymphocytes in the lymph nodes of CH dogs was significantly increased whereas that of null lymphocytes was decreased. The percentage of T lymphocytes did not differ between CH and normal dogs, however, the proportions of T lymphocyte subpopulations were significantly different. The levels of T lymphocytes expressing IgGFc receptors (T gamma) in the thymus, lymph nodes, and peripheral blood were significantly increased; whereas the levels of T lymphocytes expressing IgMFc receptors (T mu) were significantly decreased. The percentage and absolute numbers of T gamma and T mu lymphocytes cycled in CH dogs. The percentage and absolute numbers of neutrophils were greatest when that of T gamma lymphocytes was reduced. The cycles of monocytes and T gamma lymphocytes occurred in close association and a linear relationship between the levels of these cells was observed both in terms of percentage (r = 0.62; P less than 0.01) and absolute number (r = 0.67; P less than 0.05). The percentage of T gamma and T mu lymphocytes were inversely correlated (r = -0.68; P less than 0.01).