1To whom correspondence should be addressed The β-crystallins are a major constituent of the mammalian lens, where they associate into dimers, tetramers and higher order aggregates. Appropriate association of lens crystallins is important for lens transparency. To examine the associative properties of βB2-crystallin, we have expressed mouse βB2-crystallin using a baculovirus system. Recombinant mouse βB2-crystallin has an estimated monomer molecular weight of 24 kDa by SDS–PAGE, appropriate immunoreactivity and appropriate secondary structure as assessed by circular dichroism analysis. The recombinant βB2crystallin associates into a homodimer with a weight average molecular mass of 39 kDa. The βB2-crystallin homodimer has an estimated Kd of 5H10–6 M, slightly greater than that of βA3-crystallin, 0.8H10–6 M. When recombinant βB2-crystallin is combined with recombinant βA3crystallin, a heterodimer is formed within 10 min of incubation at room temperature. When equilibrium is reached in 4–6 h, approximately half of each crystallin associates into heterodimers. Subunit exchange between βB2-crystallin and βA3-crystallin occurs readily in the absence of any denaturing agents. Thus, rβA3–rβB2 heterodimer formation can occur under conditions similar to those found in the eye lens.