In one species each of the subfamilies marmosets and tamarins of the New World monkey family Callithricidae, serum levels of ascorbic acid were compared. Although the feeding regimen had been the same for both species for several years, the average concentration of serum ascorbate in the tamarins Saguinus fuscicollis (n = 21; 0.54 +/- 0.29 mg/100 ml) was only one-fifth that of the marmosets Callithrix jacchus (n = 19; 2.56 +/- 1.07 mg).