One hundred and ninety-nine patients were divided into three groups and examined for their skin reactions to dinitrochlorobenzene (DNCB). The groups consisted of those patients with rubella, those with cancer and those with dermatoses receiving long terms, small dose glucocorticosteroids. Eleven patients who did not respond to the first 0.1% challenge of DNCB two weeks after the initial sensitization responded to subsequent 0.1% DNCB challenges. Seventeen patients who were insensitive to a 0.1% DNCB challenge showed a positive skin reaction when higher concentrations (greater than 0.6%) of DNCB were used. These results indicate that among DNCB-negative patients, as determined by the present method of testing, there are those who are actually sensitized with the first dose but do not show a positive reaction.