RIPA with serum from one representative cat inoculated with a 10⁻² diluted dose of FECV RM (experiment B2). A lysate of ³⁵S-methionine labelled FCWF cells infected with FIPV strain 79-1146 was used for the immunoprecipitations. MW: molecular weight markers, of which the molecular sizes (kDa) are indicated at the left; lane 1; ascitis 9912; lane 2: anti-RM plasma; lane 3–7: serum collected from the cat one day before inoculation and at days 13, 23, 43 and 50 after inoculation, respectively. Note the relatively poor recognition of the serotype II viral spike (S) protein by the serotype I sera.