Preclinical as well as clinical studies with fibroblast interferon (IFN) are still lagging behind on those with leukocyte interferon. Its side-effects seem to be less pronounced than those of human IFN-alpha, yet it may be slightly pyrogenic after intravenous injection. Pyrogenicity of current impure preparations might for the larger part be due to impurities. Higher doses of HuIFN-beta than of HuIFN-alpha are required to obtain measurable blood titers by intramuscular injections. Since there is concern about this being due to destruction of the interferon before it has reached its target organ(s), most current clinical studies use either local (e.g. intratumoral) treatment or intravenous infusions. A study of topical treatment for acute rhinovirus infection has indicated that there is very little if any chance for fibroblast interferon to be a clinically useful substance to prevent or cure common cold. In herpetic dendritic keratitis eye drops of fibroblast interferon may be useful as such or in combination with debridement. Topical treatment of warts (multiple intralesional injections) has been shown to yield a high success rate, especially in the case of verrucae vulgares, but less so in the case of verrucae planae juveniles. Studies on condyloma accuminatum are not so far advanced as to permit a documented conclusion. Topical (intralesional) treatment of neoplastic diseases has been investigated, especially in Japan, to demonstrate that fibroblast interferon does have an antineoplastic effect in vivo. While there seems to be little doubt that local delivery does indeed cause tumor nodules to regress, the question is whether this procedure can offer a true clinical benefit to the patient. Systemic (intravenous) administration for chronic hepatitis B has been investigated further: given alone or in combination with adenine-arabinoside, fibroblast interferon seems to be able to reduce the level of viral activity. Whether this will lead to a generally accepted treatment of chronic active hepatitis is difficult to say at this moment. In treating herpes zoster in cancer patients, results have been obtained which are comparable to those found for leukocyte interferon. The practical significance of this finding must be seen in the perspective of recent developments in the chemotherapy of herpes zoster. In breast cancer patients given intramuscular injections, metastases in the skin, but not in other organs, showed alterations suggestive of an effect on tumor progression. Yet there was no true clinical benefit for the patient. In other tumors, e.g. head and neck epithelioma, no effect was seen.(ABSTRACT TRUNCATED AT 400 WORDS)