AbstractThe effect of carotenoids onin vitroimmunoglobulin (Ig) production by peripheral blood mononuclear cells (PBMNC) was examined by employing blood samples from adult volunteers and full‐term newborn babies (umbilical cord blood). Under carotenoid‐supplemented culture conditions, cells were stimulated by polyclonal stimulants, neoantigens, and a recall antigen (Ag), and IgM, IgA, and IgG levels in the culture supernatant were measuredβ‐Carotene and astaxanthin were used as representatives of carotenoids with and without vitamin A activity, respectively.Astaxanthin enhanced IgM production in response to T‐dependent Ag (TD‐Ag) anda T‐de‐pendent polyclonal stimulant. Astaxanthin also augmented IgG production in response to a recall Ag. IgA production without supplemental carotenoids was negligible for all stimuli. However, in carotenoid‐supplemented cultures, IgA production was significantly higher in response to a T‐dependent polyclonal stimulant than in unsupplemented cultures. IgM and IgA production was augmented at 10−8molli astaxanthin, whereas astaxanthin enhanced IgG production in response to a recall Ag at 10−10‐10−9molli. Similar enhancing actions of astaxanthin on IgM production were observed in cord blood mononuclear cells (CBMNC), although CBMNC produced less IgM than adult PBMNC.β‐Carotene did not have a significant effect on human Ig production. The carotenoid actions were not demonstrated under serum‐free culture conditions; serum is essential for solubilization of carotenoids. In summary, this study has shown for the first time that astaxanthin, a carotenoid without vitamin A activity, enhances human Ig production in response to T‐dependent stimuli.