A test to assess leukocyte function developed in our laboratory and based on effector mediated alterations in the cell volume was applied to study the effect of stress on the immune status of pregnant pigs. Effectors selected were prolactin, opsonized particles (Zymosan) and Zymosan together with the autologous plasma. The pigs were exposed to two stress situation: 1. fixation for an ear vein catheterization and 2. injuries by laparotomy. The investigations demonstrated that--following the stress situation 1--the effector induced alterations in the cell volume gave an indication of the susceptibility to septic inflammatory processes subsequent to the laparotomy (stress situation 2) which was carried out about 1 week after the fixation (stress situation 1). Measuring the humoral immunocompetence CRP, C3c, alpha-2-macroglobulin levels in the blood plasma had no or little prognostic value concerning the susceptibility to or the onset of the disease.