The immunologic responses of workers in a synthetic penicillin (1406059) factory (SIC-2833) were investigated. A total of 169 subjects were interviewed with reference to factors such as allergic symptoms, employment, history of atopy, and past penicillin therapy. Workers submitted to scratch tests for benzyl penicilloyl-polylysine (53608778) (PPL) and other selected agents, and to tests for hemagglutinating antibodies to penicillin. Factory air was measured for airborne penicillin and ampicillin (69534) by bioassay method. Workers were grouped by range of dust found in workroom air. Group- A was exposed to less than 0.1 milligrams per cubic meter (mg/m3) ampicillin dust while Group-C was exposed to 10.0 to 263.0mg/m3. The most frequent worker complaints included skin, upper respiratory tract, and eye irritation. No significant correlation was found between symptomatology and age of workers, last known exposure to therapeutic penicillin, or duration of factory employment. Group-A differed significantly from Group-B and Group-C on incidence of symptomatology. No significant difference existed between incidence of personal or family history of atopy in the combined population of the three groups and incidence of symptomatology. Hemagglutinizing antipenicillin antibodies were detected in the sera of 73 of 169 employees tested. There was a significant relationship between presence or absence of antibody and presence or absence of symptomatology. There was no significant correlation between the absence of detectable antibody, value of titer, and dust in the workroom areas. No evidence of local or systemic allergic reactions were detected from scratch tests with PPL. Of nine workers with dermatitis who were patch tested to reagents with which they worked, only one worker in Group-C showed a positive reaction at 48 hours. The authors recommend that a reasonable air concentration for penicillin in factories be in the range of that demonstrated in the least symptomatic group studied, with measurements below 0.1mg/m3.