Using monoclonal antibodies (MAb) and flow cytometry, Atlantic salmon neutrophils and Ig+ cells in blood and head kidney were studied in under-yearling out-of-season (0+) smolts, and 2 and 4 weeks after transfer to seawater. The parr-smolt transformation was induced using a phase advanced simulated natural photoperiod regime, and sampling of four fish was performed at regular intervals, starting on the date of the photoperiod initiation. During the freshwater period the proportion of neutrophils in the head kidney leucocytes (HKL) remained quite stable and only gradual changes in Ig+ cells were observed. In the peripheral blood leucocytes (PBL), the proportion of neutrophils markedly increased during the last month prior to seawater transfer. The most notable changes in the proportions of MAb+ leucocytes were observed in PBL after seawater transfer, with a significant increase in Ig+ cells and a significant decrease in neutrophils after two weeks in seawater. In the freshwater samples, although there were fluctuations, a decrease in the numbers of total leucocytes per millilitre blood and per gram head kidney during parr-smolt transformation was observed. The number of MAb+ cells in blood appeared to be relatively stable, while the number in head kidney tended to decrease. Following seawater transfer, the numbers of total and MAb+ leucocytes in both blood and head kidney increased markedly. The results suggest that changes in both distribution and numbers of leucocytes in peripheral blood and head kidney take place during parr-smolt transformation, and that marked changes are associated with seawater transfer. Some mechanisms possibly involved are indicated.