Zinc-deficiency occurs in well-nourished children and is mostly of dietary origin. Vulnerability to infection, diarrhea, anorexia and growth retardation is its consequence. Easy reliable techniques to diagnose Zn-deficiency are not available. We have studied for this purpose:-].salivary-Zn (0.08±0.03), urinary-Zn (480±135 ppm/day) and plasma-Zn (0.86±0.15 ppm steady decreasing for 6 hr after 100 mg egg) in volunteers (in one person no plasma decrease) .-2.erythrocyte-Zn and in vitro uptake of 65-Zn in Zn-deficient rats (dietary-Zn v uptake r=0.99).-3.the behaviour of tracer Zn in mice (65-Zn, biological T½, excretion and distribution), and humans (69m-Zn figl). For pediatric use we have developed a stable-Zn loading test (68-Zn fig2). We expect the in vitro 65-Zn uptake by erythrocytes becomes a routine test to reliably diagnose Zn-deficiency.