A pledge made this week by the nation's three major broadcast networks to reduce the amount and kind of violence shown on television is welcome but suspect. Welcome because the need to clean up the networks' act is so blatantly obvious. Suspect because of the industry's lame performance in the past.

Sociological studies keep showing that violence on TV can spawn violence in real life, particularly among young people exposed to a seemingly endless barrage of gore and mayhem. Yet the recent action by the TV networks is not the result of a sudden attack of social responsibility. Instead, it is a response to the threat of possible mandatory federal standards.Two years ago, Congress granted ABC, CBS and NBC a three-year exemption from federal antitrust standards so industry officials could sit down together and agree on some guidelines.

If they had failed to act by time the law expired this coming year, Congress might have stepped in with its own limits on violence.

Among the rules adopted by the networks are the following:

- Gratuitous or excessive depictions of violence or redundant violence shown solely for its own sake is not acceptable.

- Programs should not depict violence as glamorous or as an acceptable solution to human conflict.

- Scenes showing excessive gore, pain or physical suffering are not acceptable.

- Realistic portrayals of violence or other scenes or images that are unduly frightening to children should not be included in programs aimed at children. Scheduling of programs that contain depictions of violence should take into account the likely composition of the audience.

While such rules may certainly help, they are not the whole answer. In the first place, the three networks will police themselves. And the joint standards have no enforcement teeth. In any case, the rules are vague enough that they are open to many interpretations.

The networks realize that the problem is bigger than their programming. They have wisely suggested a conference on violence be held in Los Angeles next year for the whole enter-tain-ment industry - the major movie studios, independent producers, cable networks, syndicators. Academic, legal, medical and government leaders also would be urged to attend.

All of this seems to indicate a growing awareness of the negative consequences that can be associated with what is shown on television.

But the entertainment industry should not have limited its concern only to violence. An equally destructive practice is the use of sexual images, immoral activities and general sleaze that colors so much of television programming. The same kind of rules about violence could be applied - and should be applied - just as well to the use of sexual themes.