Acute phase proteins (APP) are an integral part of the acute phase response. Alpha 1 antitrypsin is considered to be one of the most important acute-phase protein activated by trauma, stress, or inflammatory processes. The objective of the present study was to estimate the impact of various factors (sex, month of life and sire effect) on concentration of alpha 1 antitrypsin in serum of Thoroughbred foals. A total of 624 samples, collected from 39 foals were obtained in monthly intervals from first to 16 month of life and measured by STIC method (specific trypsin inhibitory captivity). The obtained results indicated the significant impact of analyzed age periods on the AAT level. Furthermore, the variation in AAT level in analyzed periods corresponded to significant changes in foals diet and maintaining. Alpha 1 antitrypsin concentration was also affected by sire effects and sex of foals. In the most investigated age periods, the impact of sire on alpha 1 antitrypsin content in serum of his progeny has been shown. The obtained results might be useful in explanation of differences in serum AAT concentration in foals during early ontogenesis which probably is a critical period that has an influence on racing performance of young horses.