We have demonstrated marked effects of social housing condition on the growth rate of the androgen-responsive Shionogi mouse mammary carcinoma. The present study investigated the possible role of psychosocial variables in modulating the differential tumor growth rates observed. Male DD/S mice were reared individually housed (I) or in groups (G) of three or five siblings or nonsiblings. Following tumor cell injection, mice either remained in their rearing conditions (II, GG) or were rehoused (IG, GI). Effects of group size, sibling relationship, dominance status, change vs. no change in housing condition, and direction of change (individual to group or group to individual) were examined. Home cage behaviors were monitored both prior to and following tumor cell injection and rehousing. Overall, mice in the GI conditions showed faster tumor growth rates than mice in the IG conditions. Mice in the II and GG conditions showed intermediate tumor growth rates. Differences in group size and sibling relationship prior to and following tumor cell injection and rehousing had no significant influence on tumor growth rates. However, both change in housing condition and direction of change following tumor cell injection/rehousing were significant variables in modulating differential tumor growth rates. Dominance status differentially influenced tumor growth depending on whether mice experienced a change in housing; in the IG conditions, dominant mice showed faster tumor growth whereas in the GG conditions, dominant mice showed slower tumor growth than subordinate mice. Increased fighting among mice in IG compared to mice in GG conditions may play a role in modulating differential tumor growth rates.