The present invention provides methods for evaluating the regulation on prolyl 4-hydroxylase activity. The present invention also provides screening methods using the evaluation methods. Furthermore, the present invention provides methods of screening for compounds that are useful for treating or preventing fibrosis or rheumatoid arthritis. The present invention relates to methods for detecting regulatory activity on the synoviolin ubiquitination of prolyl 4-hydroxylase .alpha. subunit (P4HA1). It also relates to methods of screening for compounds with regulatory effect on the ubiquitination activity of synoviolin on P4HA1 based on these methods. Compounds discovered in the screening are useful for treating and/or preventing diseases caused by abnormalities in the prolyl 4-hydroxylase activity, such as fibrosis and rheumatoid arthritis.