This study aims to evaluate the effect of the relaxation intervention on the immune system of mothers with low birth weight babies. 60 puerperal women (control and experimental groups) with babies weighing less than 2,500g and attending the Obstetrics Department of a public hospital in Espírito Santo, Brazil, were assessed. Salivary immunoglobulin A (IgA) was used as an immunological parameter. The aim was to examine the correlation between the salivary IgA of puerperal women with low birth weight and age, behaviour and mood variables, as well as the effect of the intervention on salivary IgA. The data showed that the Relaxation intervention did not modulate salivary IgA in the experimental group after learning, internalising and practising the Relaxation technique. No correlation was found between the control variables and salivary IgA, but there was a reduction in the state of anxiety in the two groups studied, which was directly related to the type of maternal feeling towards the newborn.