(FORTUNE Magazine) – A fat Christmas bonus just isn't enough. That's the message that unions are increasingly sending to management -- and often to their own members as well. At Boeing, which has been hit by a strike of 58,000 machinists, workers are demanding double-digit wage increases instead of the big year-end bonus check that has been a tradition since 1983. That bonus amounted to 12% of total annual pay in 1986, 5% in 1987, and 5% in 1988. For employers, lump-sum payments are a lot less expensive than raises since they do not increase the cost of such benefits as pensions and sick pay, which are pegged to wages. Unions often agreed to take lump sums when their employers were having tough times some years back. But now that most manufacturers are doing better, unions in almost every industry are demanding that managements be generous throughout the year. Some already are. Last spring the United Steelworkers persuaded Bethlehem Steel to bring wages back to the levels of 1986, when the unions had agreed to an 8% pay cut in exchange for bonuses. In August the Communications Workers of America talked Ameritech into substituting wage increases for some of the bonuses in the new contracts. Union leaders are finding that their own members can be just as tough a sell as management on putting wages ahead of bonuses. Says George Kohl, research director for the communications workers' union: ''We spent a lot of time making sure our members understood this. It's not readily apparent why a $1,000 check is a bad thing.''