Human sera containing autoantibodies directed against SRP inhibit protein translocation into the ER in vitro. The secretory precursor preprolactin (PPL) was synthesized as a 35S-radiolabelled protein using a cell-free system supplemented with signal recognition particle (SRP)-depleted endoplasmic reticulum (ER) membranes and purified SRP that had been preincubated with either buffer (lanes 1 and 2) or with no additions (lanes 3 and 4) to establish normal levels of prolactin (PL) translocation into ER-derived microsomes. The specificity of protein translocation was controlled for by performing experiments lacking exogenous SRP (lanes 5 and 6) or lacking salt-washed rough microsomal membranes (RM) (lanes 7 and 8). The complete translocation of signal-sequence-processed PL into the lumen of the ER microsomes was confirmed by showing resistance to digestion by proteinase K (cf. - and + PK). To investigate the ability of distinct autoantibodies to block function, SRP was preincubated with various anti-SRP-positive sera (S), IgG (I) or Fab (F) fractions prior to protein synthesis. TL, human control serum from a healthy individual, while 19-1, 17-1, 4-2 and 25-1 are human sera containing anti-SRP autoantibodies from polymyositis patients. Serum 5–15 is from a polymyositis patient without detectable myositis autoantibodies, and serum 1–24 is from a polymyositis patient with autoantibodies directed against histidyl-tRNA synthetase. PPL, unprocessed (signal-sequence containing) preprolactin that has not been translocated into the ER microsomes. PL, fully translocated, signal-cleaved prolactin located inside ER microsomes. Samples were analysed by SDS-PAGE on 10–15% gels and by fluorography.