A series of 1H,3H-thiazolo[4,3-b]quinazolines (2a-i) were synthesized and evaluated for their in vitro antitumour activity against ca. 60 human tumour cell lines. They exhibited moderate (2c, 2d, 2f and 2g) to strong (2a, 2b, 2e, 2h and 2i) cell-growth inhibition at a concentration of 10(-4) M, but weak activity at lower concentrations. Only 1-(2,6-dichlorophenyl)-1H,3H-thiazolo[4,3-b]quinazoline (2h) possesses a significant growth inhibitory activity on 22 cell lines at a concentration of 10(-5) M.