Complementation between temperature-sensitive (ts) variants of Balb/c-3T3 defective in the G1 phase of its cell cycle was measured in the [3H]thymidine-labeling indices of the multinucleated cells during incubation at the restricted temperature (38 degrees C) following cell fusion. One ts variant from each group along the length of the G1 phase was tested for complementation. Varying degrees of complementation were observed between the 4 ts variants tested, judging by the time of entry into S-phase and the degree of synchrony attained. At least 3 complementation groups were discernible.