Standardisation of t-PA antigen and activity assays requires a t-PA standard which ideally is a single preparation suitable for all assays. Most t-PA preparations, however, are not homogeneous; they can contain several forms of t-PA: single-chain and two-chain t-PA, and two glycoforms (I and II). In this study, the behaviour of Actilyse recombinant t-PA in several assays was studied and especially the effect of the two glycosylation forms on the efficiency of the assay. In two antigen assays and a direct assay on a chromogenic substrate we observed no specificity difference for different molecular forms of Actilyse whereas, in two-step assays based on plasminogen activation, such differences were present. It is concluded that for each assay the sensitivity to different forms of t-PA should be determined, in order to be able to decide whether or not Actilyse is a suitable standard preparation.