The relationship between infectious diseases due to various pathogenetic factors and cryoglobulin production mechanisms has been investigated. Cryoglobulins have been evidenced in infections caused by very heterogeneous pathogens, i.e. leptospirosis, psittacosis, Mediterranean tick typhus, brucellosis, gram-negative bacterial septicemias, in which they had never been previously reported. In type A hepatitis a high cryoglobulin prevalence (91%) has been confirmed during the acute phase, with a rapid decrease both in prevalence and concentration in the subsequent stages of the disease. Cryoglobulins were all of type III and were mainly represented by IgM; anti-HAV-IgM antibodies have been evidenced in all but one cryoprecipitates. In non-A, non-B hepatitis a lower cryoglobulin prevalence (44.7%) was shown during the acute phase and the same fast decrease has been noted in the subsequent stages. Cryoglobulins were all of type III and in some cases polyclonal IgG was the only Ig class present in cryoprecipitates. The cryoglobulin prevalence in the acute phase of HBsAg-positive hepatitis amounted to 73.4%; all the cryoprecipitates were of type III. No correlation between the presence of cryoglobulins and HBeAg positivity or between cryoglobulins and delta agent infections was found. In all the cases studied the presence of cryoglobulins was related to the persistence of liver damage. Cryoglobulins were not found in HBsAg chronic carriers, while they have been evidenced, by a preliminary study, in 41.6% of HTLV-III antibody-positive subjects complaining of a persistent generalized lymphadenopathy without clinical or laboratory signs of liver impairment. No HTLV-III antibodies were found by ELISA method in the type III cryoprecipitates.