IT has often been suggested that the immune system may be involved in ageing1–4 Fabris et al.5 claim that such immune deficiencies are caused by inadequate stimulation of the lymphoid system by growth hormone (GH). They compared mice with inherited pituitary dwarfism (homozygotes for Snell's dwarf mutation, dw) with normal mice from the same stock. The life span of the dwarfs was only 3 to 5.5 months (mean 4.5 months) although the mean life span of their normal sibs was 20 months. The early death of the mutants was taken as a sign of premature ageing. The loss and greying of the hair, cutaneous atrophy, and bilateral cataracts which occurred in the dwarf mice from the age of 2 to 3 months were also taken as signs of ageing, as such changes only occurred in normal littermates during their last few months of life. The uptake of 3H-thymidine by several tissues, including the thymus, of a 5-month-old dwarf was less than by the corresponding tissues of a 17-month-old normal mouse. The uptakes were greater in both dwarf and normal mice aged 1 month.