Weaned pigs exposed daily to either unpredictable draught (experiment 1) or intermittent unpredictable draught (experiment 2) showed different lymphocyte blastogenic responses after mitogenic stimulation with phytohaemagglutinin (PHA) and concanavalin A (ConA). In both experiments PHA skin test responses were lower for draught exposed pigs than for control pigs and leucocyte numbers or profiles were altered compared to those of control pigs. Superoxide production and chemiluminescence of porcine granulocytes were similar for draught exposed and control animals. Furthermore, scrum globulin content did not differ significantly between pigs in the experimental and control room. The strong increase in serum γ‐globulin after the Aujeszky Disease Virus (ADV)‐challenge was the same for draught exposed and control pigs. The same held for the lymphocyte blastogenic response with ADV protein as antigenic stimulus. The present study shows the effects of climatic stress on immunological reactivity, which may reflect a homeostatic disturbance of the pig's immune system elicited by exposure to unpredictable draught.