The developmental mouse coat-colour mutations silver (si, chromosome 10) and recessive spotting (rs, chromosome 5, mapping very close to the dominant white spotting or W/c-kit locus), appear to reduce the numbers of functional melanocytes in the skin. They were studied at the cellular level by melanocyte culture. Cellular morphology, differentiation and survival appeared normal. However, both mutations were found to reduce the melanocyte proliferation rate in primary cultures, as measured by [3H]thymidine labelling indices. Two immortal si/si melanocyte lines (designated melan-si1 and melan-si2) and one rs/rs line (melan-rs) were established. Melan-si1 and melan-rs were cloned. All three immortal lines at low passage levels had doubling times significantly greater than those of our other melanocyte lines melan-a, melan-b and melan-c. Thus they retained the phenotype of slow proliferation.