A series of 1H,3H-thiazolo[3,4-a]benzimidazoles were synthesized and tested for their in vitro antitumour activity against 60 human tumour cell lines. Some derivatives exhibited both tumour growth inhibition activity and cellular selectivity. In particular, compound 8c, the most active of the series, was very active towards all cell lines at concentrations ranging from 10(-7)-10(-5) M. Compound 4a, on the other hand, was highly selective against the CNS cancer cell line.