Assuming the mString bit is a typo. I ran this code and the answer was as expected
NSMutableString * theMutableString = [[NSMutableString alloc] initWithString:@"\"Hello World!\""];
NSLog(@"%@",theMutableString);
[theMutableString replaceOccurrencesOfString:@"\"" withString:@"" options:NSCaseInsensitiveSearch range:(NSRange){0,[theMutableString length]}];
NSLog(@"%@",theMutableString);
[theMutableString release];
Output
2010-01-23 15:49:42.880 Stringtest[2039:a0f] "Hello World!"
2010-01-23 15:49:42.883 Stringtest[2039:a0f] Hello World!
So it mString was a typo in your question, then your code is correct and the problem is elsewhere. If mString is a typo in your code than that would be the issue.