The aim of this paper is to study the optical density (O.D.) values, correlated with an ELISA system to determine specific IgG4 levels against D. pteronyssinus in the serum of 20 control subjects, 35 asthmatic patients without previous immunotherapy (I.T.) and 77 patients who had variable periods of I.T. The group of non-treated patients showed specific IgG4 levels which were significantly higher than those of the control group (p less than 0.05). In this group of patients, 31.43% of the cases showed levels superior to the control group. All treated patient groups showed specific IgG4 levels which were significantly higher than those of the control group and than those of the non-treated patients, with various levels of significance (see Table IV). Of the asthmatic patients with I.T. as a whole group, 59.74% showed O.D. levels superior to the control group. The highest level of specific IgG4 was found in the patient group with the longest time of I.T. There was no significant correlation between the specific IgG4 levels with the different immunologic parameters (cutaneous reactivity, HRT, total and specific IgE). As for the clinical picture, there were no significant differences in comparing active with inactive patients during the period of study. There were no statistically significant differences in the levels observed between patients with a good clinical course and those who did not respond favourably, although all the groups of patients with a good response had higher specific IgG4 levels. This way, these antibodies may play the role of a blocking agent.