These are caterpillars of Pontia sisymbrii feeding on their host plant, the jewelflower Streptanthus breweri. You can see what appears to be a recently shed black exuviae above the smaller larva at upper left. And I'm speculating that the blurred orange-ish dot is an egg (on the blurred leaf behind the lower part of the larva on the right). I'm also wondering whether the caterpillars in this related post may be (very different looking) earlier instars of P. sisymbrii.

Streptanthus breweri is an obligate resident of rocky serpentine soils...you can see the blurred outline of its purple jewel-like flowers in the background.

Art Shapiro makes a number of interesting remarks on P. sisymbrii in his excellent "Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions"(1). In particular, on page 103 he states:

"The late-instar larva looks very un-pierid and is remarkably similar to that of the Monarch (Danaus plexippus)---ringed around the body in yellow, black, and white and, like the Monarch, fully exposed on its host plant where it stands out like the proverbial sore-thumb. This suggests either that it mimics the Monarch or that it is itself inedible and has convergently evolved warning coloration."

Concerning the hosts, Shapiro states:

"At least two of its jewelflower hosts on serpentine, S. glandulosus and S. breweri, produce orange callosities on their foliage that closely resemble eggs of this species. It has been shown in field experiments that these actually deter oviposition by females! The real eggs are laid singly near the top of the plant, and the larva often begins by feeding on foliage but later feeds primarily or exclusively on buds, flowers, and fruit."

More details on this topic can be found in:

Shapiro, A. M. (1981). Egg-mimics of Streptanthus (Cruciferae) deter oviposition by Pieris sisymbrii (Lepidoptera: Pieridae). Oecologia 48: 142–143. (PDF here)