Three serologically active fragments were identified in tryptic digests of iodinated MuLV major group-specific antigen (p30). Two of these eluted in a single zone on Sephadex G-50 and agarose gels developed with guanidine·HCl with an apparent molecular weight of 11,500. Approximately one half of the radioactivity in this zone was precipitated with antiserum to a tryptic fragment (IEFP, mol. wt. 10,300) purified by isoelectric focusing. The remaining radioactivity was still precipitable by antisera to homologous and heterologous (FeLV) p30’s. A smaller fragment (mol. wt. 2,500) not found in IEFP was apparently derived from the material in this zone. Direct radioimmunoassays and inhibition tests established that species-specific and interspecific determinants were present on each fragment. The assay with the 125I-labeled small fragment was inhibited optimally with trypsinized p30 showing an increase of 130-fold in sensitivity compared to inhibition tests with intact p