The study was carried out on 45 pigs weaned at 21 days of age with too low body weights, which is referred to as runting-stunting syndrome (RSS). The animals were divided into 3 groups depending on the cause of RSS (low weight at birth, coliform diarrhoea during sucking, or postnatal difficulties in moving). Haematological and biochemical (plasma levels of total protein, bilirubin, blood Ca⁺² and Fe⁺³ contents) parameters were determined in each group on day 7 after weaning. The results were compared with the parameters determined in normal pigs. Some pigs were immunized with sheep erythrocytes (SRBC) on day 10 after weaning. Half the immunized weaners were treated with levamisole (1 mg/kg) for 4 consecutive days to determine the immune response of animals to this immunomodulating agent. Significant changes in biochemical parameters were observed only in pigs with RSS caused by coliform infections. The immune response of pigs with RSS to the immunomodulating dose of levamisole was limited.