Hematological parameters were measured in 14 fledgling Manx shearwaters (Puffinus puffinus), with the disease puffinosis and in 10 birds that did not have the disease, on the Island of Skomer between 2 and 11 September 1991. The mean plasma fibrinogen concentration was significantly higher in the diseased birds and some of these had abnormally elevated monocyte counts. No other significant differences were observed.