BETWEEN 50 and 85 percent of all criminal cases don't go to trial but are disposed of by pleas, and a large but unknown fraction of these pleas are bargained. This means that, in return for a guilty plea of some kind, prosecutors agree either to support a favorable sentence or to drop some charges or to reduce others. To many, this collection of practices is a particularly troubling part of a system that includes capricious sentencing, strange parole decisions and procedural rules that let blameworthy people go free.

People are not merely grousing about easy treatment for white-collar or political criminals, like John Dean or bribe-pocketing congressmen; there is also a sensible fear that the legal system does a bad job of identifying the dangerous people and locking them up to protect the rest of us, whether it is the local drug pusher who bargains for a suspended sentence or murderer and novelist Jack Abbott, who won early parole and killed again. Frustration with these inadequate arrangements combines with fear of crime itself to produce failures of public confidence in the criminal justice system.

The country is in just such a period now. For example, the Court Watch Project of the conservative Washington Legal Foundation, described in this newspaper yesterday, is a nationwide effort "to hold judges, parole boards, prosecutors and other criminal justice personnel accountable for their actions." It promises a program of letter-writing, advertisements against individual judges, demonstrations, recall petitions and so forth. And efforts to curb plea bargaining or impose mandatory minimum sentences are increasingly common. None other than former vice president Mondale, in a piece on the opposite page today, complains of the dangers of excessive plea bargaining.

But plea bargaining in some form is indispensable in a world with severely limited resources for courts, prisons and lawyers. Negotiation and compromise are practical ways to conserve those resources. Moreover, a prosecutor may be uncertain about actually winning in court or may need the defendant's help in order to win some other case. The problems arise when prosecutors feel pressured to clear out a backlog or ease prison crowding by making overly lenient deals. Plea bargaining should be embraced as a mainstay of the legal system if there are guidelines to protect the public's most important interests--nondiscriminatory incarceration of the really bad or dangerous people, and more disclosure to ensure compliance with the guidelines.

There is, of course, a risk that law enforcers will exercise their discretion based on unacceptable criteria--for instance, the unpopularity of certain defendants or the political aspirations of the prosecutor or some superior. But--and here's the key problem--it's almost impossible for the public to decide whether it likes the way prosecutors, judges and parole boards make their decisions because the system is uncommunicative. Parole boards and sentencing judges offer perfunctory explanations at best, and prosecutors are scrupulously tight-lipped.

Many lawyers would hasten to point out that considerations of privacy, security and the like may excuse hiding the reasons for particular decisions. In some cases, yes. But there should be a strong presumption that officials owe explanations, and not just after-the-fact references to guidelines. Frustration and fear about crime have created demands for change in a system that is not understood. The demands will be ill-considered if they are uninformed: discretion in the criminal justice system will only be supportable if it is coupled with disclosure.