Canine cyclic neutropenia (CN), a heritable disease of Collie dogs, is characterized by the periodic cycling of formed elements of blood and the near disappearance of neutrophils every 11-12 days. Studies suggest that lymphocyte activity and regulatory mechanisms may be abnormal in CN. In this study, an immunofluorescence in the peripheral blood of normal CN Collies. Normal Collie dogs (N = 10) had 73.0 +/- 2.4% T-lymphocytes, whereas CN dogs (N = 4) had only 63.2 +/- 1.9% similar cells among peripheral blood lymphocytes (P less than 0.005). Studies on B-lymphocytes showed that normal Collie dogs (N = 10) had 17.8 +/- 2.0%, whereas CN dogs (N = 4) had 29.6 +/- 3.3% B-lymphocytes (P less than 0.005). Absolute counts for B-lymphocyte levels for 2 CN Collie dogs (N = 10) had 17.8 +/- 2.0%, whereas CN dogs (N = 4) had 29.6 +/- 3.3% B-lymphocytes (P less than 0.005). Absolute counts for B-lymphocytes were also increased in CN dogs. In a related study, daily changes in T- and B-lymphocyte levels for 2 CN Collies and 1 age-and-sex matched normal Collie were monitored over the course of a 2-week hematopoietic cycle. Similar differences in peripheral blood T- and B-lymphocyte levels, as mentioned above, were found between the CN Collies and the normal Collie. The absolute B-lymphocyte counts for CN Collies also showed a statistically significant increase (P less than 0.005) over counts for the normal Collie, and daily changes in both B- and T-lymphocyte counts were always greater for CN Collies than for normal Collies.