" That, " said the toddler to his mother as they left the cinema, " was the grossest movie I’ve ever seen. " The Farrelly brothers are back, and I’m happy to report that the constraints imposed on their œuvre by a PG rating merely serve to shift emphasis from the mangled genitalia of yore to family-friendly gags like gargantuan exploding whiteheads.

The zit in question sits on the forehead of Frank (Bill Murray), a whey-faced zookeeper with a hankering for the unhealthy. The pulsing blemish is actually a nightclub populated by all manner of cells, germs, and viruses — in fact, Frank’s entire body is a vibrant microcosm of microbes, complete with a mayoral race and a criminal-justice system. Osmosis Jones (Chris Rock) is a young white blood cell on the force; like many good cops, he’s recently seen his best efforts backfire on him. But when a malicious virus (Laurence Fishburne) intent on Frank’s destruction enters the picture, Os sees a chance for redemption. Teaming with self-important cold pill Drix (David Hyde Pierce), he sets out to kick some bacterial butt.

Despite Murray’s best efforts, the live-action interludes here are slight (and puerile in that patented Farrelly style). But in Frank’s body, animators have created a colorful and dynamic megalopolis where flatulence and phlegm are rendered with storybook whimsy.