1. The three major immunoglobulins were quantitatively determined in the serum of 112 healthy Mexico City residents by the single radial immunodiffusion method.
2. Mean levels of IgG and particularly of IgM were unusually low as compared to (a) mean levels reported from elsewhere, (b) a group of thirty-eight healthy residents of Acapulco, Mexico of similar racial background and income and (c) a ‘British’ standard reference serum.
3. It is suggested that low IgG and IgM levels in Mexico City residents may be related to its high altitude.
4. Diminished immunoglobulin levels, particularly those of IgM, might be a compensatory mechanism for the increase in blood viscosity caused by high altitude erythrocytosis.