Mutations in the SPT10 and SPT21 genes were originally isolated as suppressors of Ty and LTR (delta) insertion mutations in Saccharomyces cerevisiae, and the genes were shown to be required for normal transcription at a number of loci in yeast. Now we have cloned, sequenced, mapped and mutagenized SPT10 and SPT21. Since the spt10 mutation used to clone SPT10 resulted in very poor transformation efficiency, a novel method making use of the kar1-1 mutation was used. Neither SPT gene is essential for growth, and constructed null alleles cause phenotypes similar to those caused by spontaneous mutations in the genes. spt10 null alleles are strong suppressor mutations and cause extremely slow growth. Certain spt10 spontaneous alleles are good suppressors but have a normal growth rate, suggesting that the SPT10 protein may have two distinct functions. An amino acid sequence motif that is similar to the Zn-finger motif was found in SPT10. Mutation of the second Cys residue in this motif resulted in loss of complementation of the suppression phenotype but a normal growth rate. Thus, this motif may reside in a part of the SPT10 protein that is important for transcriptional regulation but not for normal growth. Both the SPT10 and SPT21 proteins are relatively tolerant of large deletions; in both cases deletions of the C-terminus resulted in at least partially functional proteins; also, a large internal deletion in SPT21 was phenotypically wild type.