To evaluate the effects of marginal zinc (Zn) deficiency on Zn absorption and metabolism, three groups of infant rhesus monkeys (n = 4/group) were fed from birth to 5 months of age either a regular infant formula (5 mg Zn/L) or a low-Zn formula (1 mg Zn/L). Since iron (Fe) intake may affect Zn absorption, the low-Zn formula was given without (1 mg Fe/L) or with Fe fortification (12 mg/L). At monthly intervals, Zn absorption and retention were assessed by gavage feeding with 65Zn and whole-body counting immediately after and on days 4, 7, and 11 after intubation. Blood samples were drawn before dosing for analyses of various potential markers of Zn status. Infants fed low-Zn formula had lower weight gain than controls; however, length growth was similar in all groups. 65Zn retention was considerably higher in both groups fed low-Zn formula (40%) than in the control group (20%), whereas plasma Zn levels were normal in all infants. Plasma metallothionein levels were generally very low and detectable in only 5 samples of 48; however, 4 of these were found in control infants. Neutrophil chemotaxis assessed at the end of the study was impaired in low-Zn infants compared to controls. In addition, low-Zn infants had increased levels of interleukin-2 at the end of the study. No differences were seen between the groups in hemoglobin levels, total white blood cells/absolute neutrophil counts, or plasma activities of 5'-nucleotidase or angiotensin converting enzyme. In conclusion, marginal Zn intake in infant rhesus monkeys resulted in increased Zn retention, which was not enough to completely compensate for the lower Zn intake. The higher level of iron fortification studied did not affect Zn retention significantly.