103 infants with acute obstructive bronchitis and/or bronchopneumonia were studied. IgM and alpha-1-antitrypsin values were determined by single radial immunodiffusion. In each infant the values were studies in eight serum samples. Comparison of the time of clinical recovery and x-ray clearing of the lung and the time of normalization of IgM and alpha-1-antitrypsin was also done. In infants with acute obstructive bronchitis the clinical recovery and X-ray clearing occurred on about the 13th day from the beginning of investigation. However, the normalization of IgM and alpha-1-antitrypsin values, which were significantly increased in all investigated serum samples, did not occur even on the 44th day of investigation. In infants with acute obstructive bronchitis associated with bronchopneumonia the clinical recovery occurred on the 10th day of investigation and X-ray clearing on the 14th day. IgM and alpha-1-antitrypsin values, which were also significantly increased in all investigated serum samples, were normalized on the 44th day of investigation. In group of infants with bronchopneumonia the clinical recovery occurred, approximatively, on the 12th day, whole the x-ray clearing occurred on the 17th day of investigation. IgM and alpha-1-antitrypsin values were normalized on the 18th day of investigation. In infants with acute obstructive bronchitis and/or bronchopneumonia there is significant correlation between IgM and alpha-1-antitrypsin values. This finding could be used to evaluate which of these two disease is dominant if they occur at the same time in infant.