The distribution of the enzymatic activity of dopamine beta-hydroxylase (DBH) in linear sucrose gradients was studied for a soluble fraction of the C1300 mouse neuroblastoma tumor, for the serum of tumor-bearing A/J mice, and for adrenal tissue and serum of control mice. In controls (adrenal gland and serum of A/J mice), about 75% of the DBH activity was associated with a high-molecular-weight form, denoted as DBHA, with an apparent sedimentation coefficient of 11.3 S. About 25% of the DBH activity was attributable to a slower-sedimenting species (7.1 S), denoted as DBHB. In tumor supernatants and in the serum of tumor-bearing mice, about 55% of the DBH activity was present as the 7.1 S species (DBHB), while only 35% was recovered as the high-molecular-weight form (DBHA). Approximately 5% of the activity could be attributed to a separate form, with a sedimentation coefficient of about 4.5 S. This form is designated DBHC. The ratio DBHB/DBHA is significantly higher in tumor tissue and in serum of tumor-bearing mice than in controls. The three enzymically active forms of DBH in the C1300 tumor are considered to represent the tetrameric (DBHA), dimeric (DBHB), and monomeric (DBHC) forms of the enzyme.