The purpose of this study was to study the use of saliva samples as alternative to serum for acute phase protein (APP) quantifications in pigs at field conditions. To this end, haptoglobin (Hp) and C-reactive protein (CRP) concentrations were examined in 100 animals at different ages. Sixty pigs were from a farm with chronic PRRS virus infection and 40 from a specific pathogen free (SPF) farm. A serological study was performed to obtain an overview of the immune status of animals and to evaluate possible concomitant infections in animals with PRRS infection. The results reported in this study showed that both saliva and serum samples had higher APP concentrations in PRRS pigs aged 8-9, 17-18 and 24-25 weeks in conventional herds than SPF pigs of the same age (p<0.05). In addition, increases in APP were obtained with age independently of the health status of the animals. According to the ROC analyses performed, saliva could be a better specimen than serum to quantify Hp and CRP levels in field conditions and may contribute to a more efficient detection of diseased animals at farm level.