Variation in gross cardenolide concentration of the mature leaves of 85Asclepias californica plants collected in four different areas of California is a positively skewed distribution ranging from 9 to 199 μg of cardenolide per 0.1 g dry weight with a mean of 66 μg/0.1 g. Butterflies reared individually on these plants in their native habitats contained a normal distribution of cardenolide ranging from 59 to 410 μg of cardenolide per 0.1 g dry weight with a mean of 234 μg. Cardenolide uptake by the butterflies was a logarithmic function of plant concentration. Total cardenolide per butterfly ranged from 143 to 823 μg with a mean of 441 μg and also was normally distributed. Populational variation of plant cardenolide concentrations occurs within subspecies, but the northern subspeciesA. c. greenei does not differ significantly from the southernA. c. californica. Generally higher concentrations occur in butterflies from northern populations and in females. No evidence was adduced that cardenolides in the plants adversely affected the butterflies. Low cardenolide concentrations in the leaves and the absence of cardenolides in the latex characterize bothA. californica andA. speciosa, but notA. eriocarpa. Thin-layer chromatography in two solvent systems isolated 24 cardenolide spots in the plants, of which 18 are stored by the butterflies. There was a minor difference in the cardenolide spot patterns due to geographic origin of the plants, but as in our previous studies, none in the sexes of the butterflies. UnlikeA. eriocarpa andA. speciosa, A. californica plants lack cardenolides withRf values greater than digitoxigenin. Overall, the cardenolides of bothA. californica andA. speciosa are more polar than those inA. eriocarpa. A. californica plants contain cardenolides of the calotropagenin series including calotropin, calactin, and uscharidin, and the latter is metabolically transformed by monarch larvae to calactin and calotropin. Cardenolides of this series also occur inA. vestita, andA. cordifolia from California, the neotropicalA. curassavica, and the AfricanCalotropis procera, Gomphocarpus spp., andPergularia extenso; they therefore cross established taxonomic lines.A. californica is the predominant early season milkweed in California and may be important in providing chemical protection to the spring generation of monarchs in the western United States.A. speciosa, A. eriocarpa, andA. californica each imparts distinctive cardenolide fingerprints to the butterflies, so that ecological predictions are amenable to testing.