The second virial coefficient A(2) was determined for atactic oligo- and poly(alpha-methylstyrene)s (a-PalphaMS) in cyclohexane below, at, and above Theta (30.5 degreesC) in the range of weight-average molecular weight M-w from 5.30 x 10(2) to 3.22 x 10(6). The gyration-radius expansion factor alpha(S) was also determined for the samples with M-w greater than or equal to 4.07 x 10(5). It is found that A(2) increases rapidly with decreasing M-w for small M-w because of effects of chain ends. By the use of the theory recently developed on the basis of the helical wormlike chain model with consideration of effects of three-segment interactions in addition to those of chain ends, the effective excess binary-cluster integrals associated with the chain end beads and also the effective binary-cluster integral beta between intermediate identical beads are determined as functions of the absolute temperature T. With values of beta so determined, the conventional and scaled excluded-volume parameters z and (z) over bar near the Theta temperature are calculated. The behavior of the part A(2,beta)((HW)) of A(2) without the effects of chain ends and three-segment interactions as a function of z and of alpha(S) as a function of (z) over bar is examined for the present results for a-PalphaMS along with previous ones for atactic polystyrene. It is then found that the conclusions previously derived without consideration of the effects of three-segment interactions are still valid; i.e., (1) beta is not proportional to tau = 1 - Theta/T but is quadratic in tau below Theta in contrast to the result above Theta that beta proportional to tau, (2) the result for the part A(2,beta)((HW)) below Theta is consistent with the two-parameter theory prediction, and (3) the quasi-two-parameter theory is valid for alpha(S) below Theta as well as above Theta.