The effects of a simple psychosocial manipulation, handling, on immune function in BALB/c.ByJ mice were studied. Handling for two minutes/day prior to immunization was shown to be associated with a decreased primary IgG antibody response to the protein antigen keyhole limpet hemocyanin. Handling was also associated with decreased T cell proliferative responses to Concanavalin A. No reliable changes in spleen cell number or lymphocyte subsets were found. These findings demonstrate that simply picking up an animal results in modulation of the immune response. These data are also of interest for their obvious methodological implications.