An animal model of protein-calorie malnutrition (PCM) was produced by keeping rats on a protein-free diet during the 13-day interval which followed the period of normal lactation. Malnutrition was characterized by reduction in body weight, in plasma proteins, cholesterol, haemoglobin and leucocyte counts, relative to controls from the same litter receiving a balanced diet. Acute inflammatory responses induced in these animals by carrageenin or staphylococci suspensions, differed markedly from those evoked in matching controls: the resulting oedema developed more slowly and yet persisted for longer intervals; Evans blue, injected i.v., leaked into the inflamed areas in smaller quantities; reduced cell migration to the site of the lesion was observed. The slow development of the response was attributed to the decreased levels of circulating proteins and its persistency to a defective drainage of exuded material. It is concluded that PCM can impair the capacity of response to noxious stimuli, thus rendering the animals more susceptible to infections.