The blastogenic effect of phytohemagglutinin (PHA) on peripheral lymphocytes from six Rocky Mountain bighorn sheep was examined for 3 months following their establishment in captivity. Lymphocytes survived well in culture for 6 days and responded to PHA with morphologic transformation to blastoid cells which were readily identifiable by fluorescent microscopy following acridine orange staining. Transformation of lymphocytes from animals at the time of initial establishment was low but as the animals adjusted to captivity, transformation increased markedly. These observations appeared to reflect predominantly cellular changes since serum samples collected throughout the study did not differ in their ability to support transformation.