The mitogen induced blastogenic response of lymphocytes from normal dogs and dogs with generalized demodicosis (GD) was measured by the ethidium bromide (EB) fluorescence assay. Serum from GD dogs significantly suppressed the in vitro reactivity to Con A of peripheral blood lymphocytes (PBL) from normal dogs and GD dogs, however with a different percentage of suppression 40.6 and 81.2%, respectively. As a result, the degree of lymphocyte blastogenesis suppression in GD dogs did not parallel the immunosuppressive potency of their serum (Tab. IV). The data indicate that PBL obtained from GD dogs did not respond to Con A as well in the presence of serum from normal dogs as did PBL from normal dogs (Tab. IV). In one third of examined GD dogs a similar situation was described also by Hirsh et al. (1975). The basis for this cellular modified response is unknown. It does not appear that the age or the chronicity of the disease are related to this observation. Further studies are necessary to elucidate this relation. The GD dogs showed not only a significant depression of the lymphocyte response to Con A but also enhancement of the ability of unstimulated cells to proliferate was also observed (Tab. IV). Similar observation was reported by others (Barriga et al., 1992). The meaning of this is not clear at present. This finding is discussed in the light of proposed different effects of the parasite or the host's reactivity to the parasite on different subsets of lymphocytes. No significant difference of PBL responsiveness to Con A between healthy dogs with respect to the age (Tab. III) and the time of examination (compare results in Tabs. I and IV) was observed. Autologous serum showed a better responsiveness of normal canine lymphocytes to Con A than fetal calf serum (FCS). It is suggested that the use of FCS might lead to an erroneous judgement (Tab. I). Both lectins, Con A and PHA induced cell proliferation of healthy dogs in very similar amount (Tab. II). Our results indicated that EB fluorescence assay is a useful method for detection a respondence of canine lymphocyte blastogenesis.