These studies were undertaken to investigate the effects of increasing or decreasing IGF-1 levels on aspects of immune function in rats. Female dwarf rats were treated with recombinant human IGF-1 or with a potent sheep anti-IGF-serum. Body weight, thymus weight and spleen weight increased with IGF-1 treatment (p < 0.001), while there was no effect of anti-IGF-1 treatment when compared with the appropriate normal sheep serum (NSS) treated controls. IGF-1 treatment significantly decreased WBC and RBC counts, but increased the ratio of CD4(+):CD8(+) T-cells. Anti-IGF-1 serum had no effect on these parameters compared with MSS. However anti-IGF-1 was associated with increased T-cell numbers, decreased natural killer cells, and enhancement of the animals' ability to produce specific IgG in response to injection of keyhole limpet haemocyanin (KLH). These results indicate that IGF-1 may suppress immune function although increasing the size of immune organs such as spleen.