We have adapted a [3H]mannose suicide selection to identify mutations in additional genes which function in the early part of the yeast secretory pathway. Thus far this protocol has led to the identification of two new genes which are implicated in this process, as well as additional alleles of previously identified genes. The new mutants, bet1 and bet2, are temperature sensitive for growth and protein transport. Thin section analysis has revealed the accumulation of a network of endoplasmic reticulum (ER) at the restrictive temperature (37 degrees C). Precursors of exported proteins that accumulate in the cell at 37 degrees C are terminally core glycosylated. These observations suggest that the transport of precursors is blocked subsequent to translocation into the ER but before entry into the Golgi apparatus. The bet1 and bet2 mutants define two new complementation groups which have the same properties as previously identified ER-accumulating mutants. This and previous findings (Novick, P., C. Field, and R. Schekman, 1980, Cell, 21:205-215) suggest that protein exit from the ER and entry into the Golgi apparatus is a complex process requiring at least 11 genes.