Summary: When 52 ragweed-sensitive patients were observed during 3 to 5 pollinating seasons and the number of “intensity-hours” of hay fever for each was compared with the amount of thermostable antibody in his circulation during the season, a close parallel was found to exist between the amount of antibody and the degree of clinical insusceptibility in 79 per cent. Another 19 per cent of the group showed the correlation in all years but one, whereas one patient failed to show any connection between the two factors. The degree of clinical resistance which was associated with a given amount of thermostable antibody in the circulation varied decidedly with the individual, some requiring much more antibody than others for effective immunity. It is suggested that pollen-sensitive patients be treated according to their particular immunological needs, rather than empirically as in the past. The immune state of such individuals may be used as a guide to treatment, especially if this be determined periodically during the first year of successful therapy. The conjunctival reaction, obtained with threshold-amounts of antigen, serves as a simple index to immunity.