Type IV collagen is a major component of basement membranes. We have characterized 11 mutations in emb-9, the alpha1(IV) collagen gene of Caenorhabditis elegans, that result in a spectrum of phenotypes. Five are substitutions of glycines in the Gly-X-Y domain and cause semidominant, temperature-sensitive lethality at the twofold stage of embryogenesis. One is a glycine substitution that causes recessive, non-temperature-sensitive larval lethality. Three putative null alleles, two nonsense mutations and a deletion, all cause recessive, non-temperature-sensitive lethality at the threefold stage of embryogenesis. The less severe null phenotype indicates that glycine substitution containing mutant chains dominantly interfere with the function of other molecules. The emb-9 null mutants do not stain with anti-EMB-9 antisera and show intracellular accumulation of the alpha2(IV) chain, LET-2, indicating that LET-2 assembly and/or secretion requires EMB-9. Glycine substitutions in either EMB-9 or LET-2 cause intracellular accumulation of both chains. The degree of intracellular accumulation differs depending on the allele and temperature and correlates with the severity of the phenotype. Temperature sensitivity appears to result from reduced assembly/secretion of type IV collagen, not defective function in the basement membrane. Because the dominant interference of glycine substitution mutations is maximal when type IV collagen secretion is totally blocked, this interference appears to occur intracellularly, rather than in the basement membrane. We suggest that the nature of dominant interference caused by mutations in type IV collagen is different than that caused by mutations in fibrillar collagens.