The purpose of these investigations was to assemble and compare data which might illustrate a functional relationship between phytohaemagglutinin-induced transformation of equine lymphocytes in vitro and the haematological profiles of the lymphocyte donors. Statistically significant correlation between transformation and a haematological parameter was taken as evidence that both may be part of a common regulatory system. There was no evidence that transformation in vitro was affected by artefactual variation in the cellular composition of the cultures. Transformational behaviour of lymphocytes in autologous plasma showed significant correlation with donors' total leucocyte, neutrophil and T cell count. Different batches of donor plasma varied in ability to support the transformation and this ability was found to correlate significantly with the total leucocyte, eosinophil, lymphocyte and T cell count of the original blood. It is concluded that previously reported variation in transformation ratios of normal equine lymphocytes is attributable to regulatory mechanisms affecting circulating blood as a whole and that helminth parasitism may be among the primary causal agents which modify the mechanism.