Once you have the nerve toxin, the alarm will sound. Zap both lab rats and then enter the lab to grab the toxin. From here, Steve will find another lab rat and this will encourage the locked lab rat to open the door.

The lab rat with the nerve toxin is locked inside his lab and won't open the door.

Zap them when they exit (use Michael's special ability to get them both, otherwise your colleagues will shoot one of them down) and then enter the next corridor towards the door with the security lock. Take cover and wait for the two guards to walk out. If it truly belongs in troubleshooting, please move this.Continue ascending the lab halls and corridors until you reach the security doors. P.S.- Wasn't sure whether to put this in the troubleshooting or mission section, as it deals with a mission-related bug, but I figured users would likely turn to 'mission help' in such a situation. And if anyone else knows something, please post it so we can help the community. I'm not entirely sure whether this problem is widespread, but I couldn't find much info Googling the problem (main issue being that the phrase 'monkey business' is used way too often), but maybe posting my findings here will help someone else out, and that makes it worthwhile.

I also ran into the main room to kill the final guard, instead of killing him in the hallway, but again, I did this on one of my failed runs, as well as me seeing a video walkthrough of the mission where the player killed the last enemy while in the hallway. The only things I did differently were actually stunning the patrolling guards instead of letting them pass, and switching to a lethal gun immediately after stunning the scientists (though I tried that, as well as killing them, during my many failed runs). So after maybe 20 runs through the post-stealing-the-gas section with the same result (I even tried stunning as many of the army as I could for kicks, same result), I cancelled the mission and restarted it, this time switching to Franklin (as was suggested by another person online) instead of starting it with Michael. Now, after failing many times, I probably could've skipped the section, but that's not how I wanted to beat the mission. Killing either agent (as was suggested by someone else when I was researching a fix) fails the mission as well. No sign of additional troops, and running down the road fails the mission for abandoning Dave and Steve. However, none of the agents say a word and stay in their 'cover' position. However, after killing them all, Steve is supposed to make a comment and give Michael the waypoint for the refrigeration unit. In the mission, after you steal what you came for and try to leave, police and military block your path.

Gta 5 monkey business save game