In this study we evaluated the differences in plasma levels of some glycohydrolases of lysosomal origin that appear to be the most interesting for possible usefulness for diagnosis (N- acetyl -ß- d -glucosaminidase , ß-d-glucuronidase, α-d-galactosidase, ß-d-galactosidase, αl-fucosidase and α-d-mannosidase) in a general population of 417 subjects, as related to age and sex and also to body mass and to some habits, such as smoking and consumption of alcohol. The enzymatic activities were assayed by fluorimetric techniques with 4-methylumbelliferyl-glycosides as substrates. Particular attention was given to some technical aspects. Enzymatic activity was preserved by addition of ethylene glycol and stable liquid material was employed for calibration purposes. Blood was sampled rigorously at the same time of day and all the samples were obtained within a short period of time to exclude effects of the circadian and circannual rhythms. ß-Glucuronidase levels were the most affected by sex and body mass. ß-d-Galactosidase was not affected by differences in age, sex, body mass or by smoking, but appeared to be the most sensitive to modification by alcohol consumption. The data in this report emphasize that, whenever changes or differences in the levels of lysosomal enzymes in body fluids are studied, it is essential to have a reference population rigorously correlated with the study population. When possible, repetitive measurements in the same subject could better indicate a clinical trend.