The human mucolipidoses (ML) are characterized by abnormal activities and abnormal electrophoretic patterns of fibroblast lysosomal hydrolases. These altered mobility patterns can be used to confirm the clinical diagnosis of the four mucolipidoses. The mobility patterns of one nonlysosomal and seven lysosomal enzymes were tested in fibroblasts from two ML I (sialidosis type 2, infantile), fifteen ML II (I-cell disease), eight ML III (pseudohurler polydystrophy), and one ML IV patients. A single sialidosis type 2, juvenile, line was also examined. Characteristic mobility patterns were found which identify each of the four mucolipidoses. Both the ML I and sialidosis type 2 juvenile lines displayed anodal mobility patterns, but distinct differences between the two disorders were observed. Lysosomal hydrolases from ML II lines demonstrated reduced activities or had altered mobilities. Differing electrophoretic patterns demonstrated the presence of at least two groups within the clinical phenotype diagnosed as ML II, indicating heterogeneity. The ML III lines showed normal electrophoretic patterns for most lysosomal hydrolases. The ML IV line expressed normal mobilities for every enzyme studied, with a single exception. The electrophoretic patterns of only beta-hexosaminidase, acid phosphatase-2, alpha-galactosidase, and esterase A4 were sufficient to identify and distinguish the different mucolipidosis types. Electrophoretic variation was also seen in liver but not kidney extracts from three ML II patients. beta-Hexosaminidase and alpha-mannosidase B secreted into the medium by ML II and ML III fibroblasts had mobility patterns different from normal and from their intracellular patterns. These data suggest that the mucolipidoses are genetically distinct with heterogeneity within them.