Transcriptional and translational fusions were made between the reading frame coding for beta-D-glucuronidase and sequences of either a constitutively expressed rice gene (GOS2) involved in initiation of translation or a light-inducible rice gene (GOS5). The transient expression of the fusions was studied via particle bombardment of seedling tissues of rice, perennial ryegrass and barley. Furthermore, the results of transient and stable expression were compared for cell suspensions of four rice varieties, one barley variety and one perennial ryegrass variety. The GOS2-gusA fusions were active in all three monocots studied. Best results were obtained for a construct having both a transcriptional and a translational fusion as well as intron and exon sequences (PORCEHyg). The level of GUS activity was in the range of activities as obtained by the 35S CaMV promoter transcriptionally fused to gusA. The gusA fusion with the light-inducible gene (GOS5) was active in green seedling tissues of all monocots studied. Also a weak expression compared to the GOS2 constructs was found in stably transformed rice callus. The gusA fusions with the mannopine synthase promoters 1' and 2' of the TR-DNA were transiently expressed at lower levels in cell suspensions than PORCEHyg. For stably transformed rice callus the expression of the GOS2-gusA fusion often decreased during prolonged subculture. This decrease in GUS activity and the various GUS-staining phenotypes of transgenic calli are explained by the presence of different cell types in the suspensions used and in the calli. It is presumed that the nature of the cells and their relative contribution in the calli change drastically upon further subculture.