The effects of postnatal manipulations on different immune parameters were investigated in adult female mice. Postnatal stress consisted of a 15‐min daily exposure to clean bedding (temperature Mayntained at 35°C) for the first 2 weeks of life in the absence of the mother but in the presence of littermates. Controls were unhandled until weaning. At 60 days of age, female mice stressed during postnatal development showed enhanced immune reactivity as assessed by NK‐cell activity and T‐cell mitogenesis in comparison with unhandled mice. By contrast, B‐lymphocyte proliferation induced by lipopolysaccharide (LPS) was not affected by alterations of postnatal environment. Furthermore, the association between immune reactivity and behavioral lateralization observed in adult mice was not altered by postnatal stress.© 1994 John Wiley & Sons, Inc.