Plasma alkaline phosphatase (ALP) activity was determined in 4 laying strains (White Leghorn, New Hampshire, Native Brown and Native Barred) at 48 and 54 weeks of age. Birds were fed 1 of 2 isoenergetic diets with calculated crude protein contents of 152 and 181 g/kg. 2. No significant differences in plasma ALP activity were noted between the strains, although significant differences in laying performance between strains were evident. 3. There was no response in plasma ALP activity to the high dietary protein level in any strain. Depressed plasma ALP activity was associated with increasing age of birds. 4. The results failed to confirm the suggestion that ALP activity is related to egg production of the laying hen.
