Characteristics of attenuated lines obtained by selection for precocious development in the chicken. [McDonald V, Shirley MW, Bellatti MA.] Abstract: Both the Weybridge strain and a mixture of six laboratory and field strains of Eimeria maxima have been attenuated by selection for early maturation of oocysts during serial passage in chickens. The prepatent times of the resultant precocious lines produces after selection were reduced from approximately 120 hr to less than 107 hr, and their reproduction and pathogenicity were less than those of the parent strains. Chickens which had been inoculated with a small number of oocysts of the Weybridge precocious or mixed field isolate precocious lines were immune to subsequent challenge with the parent strains. In a comparison made between the endogenous development of the precocious lines and their parent strains, it was found that asexual multiplication of the precocious lines was reduced, due to the earlier onset of gametogony. The results of this study also indicate that the parent (i.e., normal) strains of E. maxima probably undergo a minimum of four generations of schizogony.