A transgenic murine embryonic stem (ES) cell lineage expressing enhanced green fluorescent protein (EGFP) under the control of alpha-myosine heavy chain (alpha-MHC) promoter (palpha-MHC-EGFP) was used to investigate the effects of (thio)urea and cinchona alkaloid derivatives on cardiomyogenesis. The screening of the compounds yielded cardiomyogenesis inducing substances with good (IV-5, V-4) to very good activities (II-16, IV-8), as determined by a 50 to 80 % increase in the EGFP fluorescence compared to untreated cells. Time-dependent screening approaches in which compounds were added at different developmental stages of the ES cells appeared to be of limited suitability for the identification of potential cellular targets.