A 24000-dalton protein [yeast eukaryotic initiation factor 4E (eIF-4E)] was purified from yeast Saccharomyces cerevisiae postribosomal supernatant by m7GDP-agarose affinity chromatography. The protein behaves very similarly to mammalian protein synthesis initiation factor eIF-4E with respect to (i) binding toand elution from m7GDP-agarose columnsand (ii) cross-linking to oxidized reovirus mRNA cap structures. Yeast eIF-4E is required for translation as shown by the strong and specific inhibition of cell-free translation in a yeast extract by a monoclonal antibody directed against yeast eIF-4E. e binding of eukaryotic ribosomes to mRNA is a multistep process involving mRNA cap recognition, scanning of the 5'nontranslated region, and AUG start codon selection [for reviews, see Safer & Anderson (1978), Revel & Groner (1978), Kramer & Hardesty (1980), Maitra et al.(1982), and Perez-Bercoff (1982)]. These steps are catalyzed by 6 or more eukaryotic initiation factors (elF’s) 1 composed of a total of about 15 polypeptides. One of the earliest reactions, the recognition of the mRNA cap structure by the ribosome, is mediated by the factors eIF-4A (Grifo et al., 1982; Edery et al., 1983), eIF-4E (also termed 24K-CBP or CBP-I; Sonenberg et al., 1978, 1979), eIF-4F (also termed CBP complex or CBP-II; Tahara et al., 1981; Edery et al., 1983; Grifo et al., 1983), and very likely eIF-4B (Grifo et al., 1982; Edery et al., 1983). It requires the hydrolysis of ATP (Kozak, 1980) and may involve melting of RNA secondary structure in the 5'proximal region (Sonenberg et al., 1981, 1983). Cap recog-nition is a regulatable step as has been clearly demonstrated for poliovirus-infected HeLa cells, where the factor eIF-4F is inactivated early in infection leading to a shut-off of host mRNA translation [for a review and references, see the introduction to Sonenberg et al.(1983)]. At present, we know very little about the functions of individual initiation factorsand their subunits in cap recognition and cap binding and how their activities are regulated in uninfected cells. To answer these questions, we have begun to study mRNA binding to ribosomes in the yeast Saccha-romyces cerevisiae where biochemical approaches can be combined with powerful genetic approaches. Here, we report on the isolation from S. cerevisiae of one of the factors involved in mRNA cap recognition, the initiation factor eIF-4E. f This work was supported by Grant 3.117-0.81 from the Swiss National Science Foundation to HT and by grantsfrom the Medical Research Council and the National Cancer Institute of Canada to NS MA is a recipient of an EMBO long-term fellowship, and IE is a recipient of a predoctoral fellowship from the Cancer Research Society (Montreal).