diff --git a/src/ReplaySubject.ts b/src/ReplaySubject.ts
index 0ab64ee603..71655aed01 100644
--- a/src/ReplaySubject.ts
+++ b/src/ReplaySubject.ts
@@ -41,7 +41,7 @@ export class ReplaySubject<T> extends Subject<T> {
     } else if (this.isStopped) {
       subscription = Subscription.EMPTY;
     } else {
-      this.observers.push(subscriber);
+      this._observers.push(subscriber);
       subscription = new SubjectSubscription(this, subscriber);
     }
 
diff --git a/src/Subject.ts b/src/Subject.ts
index b3dacf0d80..ba223ceca1 100644
--- a/src/Subject.ts
+++ b/src/Subject.ts
@@ -25,15 +25,23 @@ export class Subject<T> extends Observable<T> implements ISubscription {
     return new SubjectSubscriber(this);
   }
 
-  observers: Observer<T>[] = [];
-
-  closed = false;
+  protected _observers: Array<Observer<T>> = [];
+  public get observers(): ReadonlyArray<Observer<T>> {
+    return this._observers;
+  }
 
-  isStopped = false;
+  protected _closed: boolean = false;
+  public get closed(): boolean {
+    return this._closed;
+  }
 
-  hasError = false;
+  protected _isStopped: boolean = false;
+  public get isStopped(): boolean {
+    return this._isStopped;
+  }
 
-  thrownError: any = null;
+  protected hasError: boolean = false;
+  protected thrownError: any = null;
 
   constructor() {
     super();
@@ -69,34 +77,34 @@ export class Subject<T> extends Observable<T> implements ISubscription {
     }
     this.hasError = true;
     this.thrownError = err;
-    this.isStopped = true;
+    this._isStopped = true;
     const { observers } = this;
     const len = observers.length;
     const copy = observers.slice();
     for (let i = 0; i < len; i++) {
       copy[i].error(err);
     }
-    this.observers.length = 0;
+    this._observers.length = 0;
   }
 
   complete() {
     if (this.closed) {
       throw new ObjectUnsubscribedError();
     }
-    this.isStopped = true;
+    this._isStopped = true;
     const { observers } = this;
     const len = observers.length;
     const copy = observers.slice();
     for (let i = 0; i < len; i++) {
       copy[i].complete();
     }
-    this.observers.length = 0;
+    this._observers.length = 0;
   }
 
   unsubscribe() {
-    this.isStopped = true;
-    this.closed = true;
-    this.observers = null;
+    this._isStopped = true;
+    this._closed = true;
+    this._observers = null;
   }
 
   protected _subscribe(subscriber: Subscriber<T>): Subscription {
@@ -109,7 +117,7 @@ export class Subject<T> extends Observable<T> implements ISubscription {
       subscriber.complete();
       return Subscription.EMPTY;
     } else {
-      this.observers.push(subscriber);
+      this._observers.push(subscriber);
       return new SubjectSubscription(this, subscriber);
     }
   }
diff --git a/src/SubjectSubscription.ts b/src/SubjectSubscription.ts
index c7080ed6f5..8ca2e5fa68 100644
--- a/src/SubjectSubscription.ts
+++ b/src/SubjectSubscription.ts
@@ -22,7 +22,7 @@ export class SubjectSubscription<T> extends Subscription {
     this.closed = true;
 
     const subject = this.subject;
-    const observers = subject.observers;
+    const observers = (subject as any)._observers;
 
     this.subject = null;