A candlelit vigil for 21 people who died in the Birmingham pub has been held at Birmingham Cathedral.
Two bombs exploded in the Tavern in the Town and The Mulberry Bush pubs on 21 November 1974.
Six men were jailed, but their convictions were quashed in 1991.
The Justice 4 the 21 group said it held the vigil to "heighten attention" given to their campaign for a public inquiry into the bombings.
During the service, members of the group lit 21 candles in memory of those who died.
The group said it had a 4,000-name petition calling for the public inquiry which it hopes to hand in at Downing Street early next year.
Julie Hambleton, whose sister Maxine was killed in the bombings, said: "We will continue fighting until we get a breakthrough.
"It appears that the authorities seem to believe that our friends and family members, my sister and the other 20 victims, are expendable, that their lives aren't worth anything.
"They were murdered en masse and no-one has been brought to justice."
The Court of Appeal ruled in 1991 that the convictions of six men from Northern Ireland, known as the Birmingham Six, were unsafe because they were based on unreliable forensic evidence and "confessions" obtained using violence.
No-one else has since been charged over the bombings.
In July, the West Midlands Counter Terrorism Unit said it would work with forensic experts to review evidence to see "what opportunities we might have to resolve the case".
It said the consultation would take "several months" because officers would have to look at a "vast number" of files.
Taking part in a debate on BBC WM earlier this month Bob Jones, the new West Midlands police and crime commissioner, said he backed the group's calls for the investigation to be reopened, along with the other candidates.
Mr Jones said the lack of justice for the families was "a continued stain on West Midlands Police".