Lysates of various mammalian RSV-transformed cells labelled with 35S-methionine were tested for the presence of gs proteins and for the size of these molecules by means of antisera, using sodium dodecyl sulphate--polyacrylamide slab gel electrophoresis. In no case was the presence of gs proteins detected in the analyzed immunoprecipitates; only in RSH-mix cells a quantitative difference in the contrast of a 70 000-molecular weight protein was found. The autoradiograms of simultaneously tested immunoprecipitates of lysates of RSV-transformed chicken cells clearly showed the presence of p27 and after a short-term labelling also of the gs precursor molecule (pr76). The cause of the failure in detecting gs proteins by the used method in two mammalian RSV-transformed lines, in which the gs antigens were previously detected by complement fixation test, are discussed.