We have sodium dodecyl sulfate-polyacrylamide gel electrophoresis to analyze the virion proteins of an N- and a B-tropic C-type virus derived from the BALB/c mouse and 21 putative recombinants, designated XLP-N viruses, obtained from seven crosses between these N- and B-tropic viruses. All the XLP-N viruses are N-tropic but posses the XC plaque morphology of their B-tropic virus parent. Three virion proteins, p15, p30, and gp70, of the parental viruses each differ in electrophoretic mobility. Two recombinants were found that possess a p15 that comigrates with p15 of the B virus; 19 possess a p15 that comigrates with N virus p15. Sixteen recombinants possess a gp70 that migrates like the gp70 of the B virus: four have gp70 with an electrophoretic mobility like that of the N virus gp70. All 21 recombinants possess a p30 that comigrates with p30 of their N virus parent. Given the origin and phenotype of XLP-N viruses, these results would seem to provide good evidence that these viruses are recombinants.