Sixteen lower-middle-class families, comprising 100 persons, were intensively studied for one year, with systematic throat cultures for beta-hemolytic streptococci, periodic measurements of antistreptolysin O titer, and clinical evaluation of all illnesses; the results of those observations were compared to certain dependent variables of host, agent, and environment. The factors that seemed to play an important part in determining whether a given person acquired a streptococcus, became ill with this acquisition, or developed a subsequent increase in antistreptolysin O were age, season, closeness of contact with an infected person as measured by sleeping arrangements, acute or chronic family stress, and penicillin treatment. No relationship was found between streptococcal episodes and the number or type of streptococci present, sex of the patient, the presence or absence of tonsils, an allergic history, changes in weather, type of housing, or family size. Further evidence for the multiple causation of beta-hemolytic streptococcal disease has been obtained, although the mechanisms through which these factors exert their influence are not clear.