Babesia bovis rapidly incorporated [32P]orthophosphate into more than 30 polypeptide chains, the majority of which appeared to be membrane or particle bound. Phosphorylation occurred more rapidly in free parasites than in parasitized erythrocytes, suggesting the acceptor polypeptides were either babesial in origin or erythrocyte proteins were intimately associated with the parasite. Some characteristics of the phosphorylation system are described. The phosphorylation was little affected by dibutyryl cAMP or dibutyryl cGMP but was strongly inhibited by the calcium ionophore A23187, which also inhibited protein synthesis. Two dimensional electrophoresis of B. bovis extracts after pulse labelling with [35S]methionine or [32P]orthophosphate showed that all polypeptides phosphorylated in a 30 min period were also at least partially synthesized de novo within that period, suggesting that the phosphorylated species were babesial in origin.