Acetic acid derivatives of tricyclic systems, such as 6,11-dihydro-11-oxodibenzo[b,e]thiepin, 4,10-dihydro-4-oxo-thieno[2,3-c][1]benzothiepin, dibenzo[b,f]thiepin, dibenz[b,f]oxepin, etc., were synthesized and assayed for antiinflammatory activity. One of the compounds, 6,11-dihydro-11-oxodibenzo[b,e]thiepin-3-acetic acid (52), was chosen for evaluation in man on the basis of high antiinflammatory activity in both short- and long-term animal assays and a low gastric irritation liability in rats and dogs.