The effects of the long term storage of canine blood at 4 to 6 degrees C in PVC-bags containing CPDA-1 as a stabiliser on platelet aggregation and platelet counts were investigated. Aggregation induced by collagen, adenosine diphosphate or a calcium ionophore was preserved well during the first six hours, but there was then a decrease of 24 to 46 per cent in the ability of the platelets to aggregate, after which during the next three to four weeks of storage there was no further decrease in their aggregation properties. The formation of aggregates of platelets reduced the numbers of platelets counted as single thrombocytes by more than 30 per cent during the first four days. The numbers of platelets recorded varied widely with the counting method used (counting chamber or automatically) and with the ethylenediamine tetra-acetic acid content of the dilution fluid.