Disruption of Rauscher leukemia virus (RLV) with low levels of Nonidet P-40 yielded "immature" cores. These cores have a diameter of about 920 A, as opposed to the 1300-A diameter of RLV, possess knob-like protuberances, and contain a concentrically coiled internal strand apposed to the core shell. The two major polypeptide components of immature cores are (i) p30, the 30,000-dalton group-specific antigen, and (ii) a polypeptide that has the size and antigenic characteristics of P70, the 70,000-dalton precursor protein of the group-specific antigens of murine leukemia virus. Disruption of RLV at high ratios of Nonidet P-40 to virus yielded "mature" cores. These cores have an average diameter of 850 A, a smooth proteinaceous perimeter, and a collapsed internal strand, and they contain predominantly p30. Treatment of RLV with low levels of Nonidet P-40 for 16 hr at 22 degrees yielded cores that showed (I) a 70% decrease in the number of immature forms and concomitant increase in the number of mature forms, (II) a 60-90% decrease of P70, and (iii) a 30% increase in a 40,000- to 42,000-dalton protein. These results suggest that maturation of RLV cores is accomplished by cleavage of P70.