The levels of circulating 25 OH-D were determined by a direct radio-competition methods both in normal subjects and in subjects with various pathological conditions. In normal subjects, the average level of 25 OH-D was higher in summer (42.3 ng/ml) than in winter (29.1 ng/ml), P less than 0.005. Monthly variations in the 25 HO-D levels were found in relation to insolation The level of 25 OH-D was practically normal in osteoporosis (28.9 ng/ml), clearly lower in the mixed forms called "osteoporomalacia" (13.5 ng/ml, P less than 0.005) and very low in osteomalacia (5.8 ng/ml, P less than 0.001). In cases of cortisone osteopathy the average level was 22.8 ng/ml (NS). The level of 25 OH-D was also found to be lower in hepatic cirrhosis (11.7 ng/ml, P less than 0.01), in subjects treated with anticonvulsants (P less than 0.01), and in the course of hyperparathyroidism (P less than 0.002). There was no corelation between the level of 25 OH-D and calcaemia, phosphoraemia, circulating immunoreactive parathyroid hormone, or the relative osteoid volume. In contrast, there seemed to be a good correlation with the level of alkaline phosphatasaemia. The level of 25 OH-D was also determined in 4 subjects with vitamin-resistant osteomalacia: in 3 cases hepatic hydroxylation seemed normal, indicating the possibility of a subsequent disorder of vitamin D metabolism; in one case the absence of hepatic hydroxylation was noted.