The MHC class II-associated invariant chain behaves as a resident endoplasmic reticulum protein in the absence of class II molecules. In humans, two predominant forms exist; one, p35, differs from the other, p33, by an N-terminal cytoplasmic extension of 16 amino acids that contains a strong endoplasmic reticulum-retention signal. Here we show that one mechanism for retention of p33 is its association with p35 in mixed invariant chain trimers. However, even for p33 homotrimers transport from the endoplasmic reticulum is inefficient. In an MHC class II-positive B cell line, the formation of invariant chain trimers is rapid and is the first intermediate in the assembly of a nine-chain alpha beta-invariant chain complex. With time, three higher molecular weight complexes are progressively formed. These correspond to an invariant chain trimer with one alpha beta dimer, two alpha beta dimers, and three alpha beta dimers, respectively. No free alpha beta dimers are detectable early in biosynthesis. However, beginning at 2 h of chase, alpha beta dimers begin to appear concomitant with the disappearance of the completely assembled alpha beta-invariant chain complex. This conversion is virtually complete by 4 h, and presumably reflects the proteolytic degradation of the invariant chain component of the alpha beta-invariant chain complex and the generation of endosomal alpha beta dimers capable of binding antigenic peptides.