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Transverse lines (Harris lines) in long bones are density anomalies thought to be associated with intervals of growth disturbance and subsequent recovery. In this study, radiographs of infant rhesus monkeys, Macaca mulatto, were examined for evidence of transferse lines in the distal ends of the left radius. The objective was to document the frequency of early postnatal transverse line formation, i.e. the ap­pearance of birth lines. Other variables considered were animal sex and birthweight. The effects of different infant diets were also evaluated to determine whether the availability of protein affected the frequency of line formation.Thirty monkeys were placed on a protein-restricted diet at birth. Data for the experimental animals were compared with that of controls (n = 55) maintained on a diet adequate in protein. A higher mean number and higher percentage of transverse lines occurred in controls than in protein-restricted infants. No sex differences were present. Animal birthweight was not correlated with the occurrence of transverse lines.At age 120 days, 10 protein-restricted animals were placed on the higher protein diet of controls. Sequential x-rays were then examined to determine whether this transition was followed by the appearance of transverse lines. Recovery from the protein-restricted diet was not reflected by the formation of a line.